Communication network, path setting method and recording medium having path setting program recorded thereon

ABSTRACT

To prevent load of route calculation from being centralized in part of units. A node calculates routes of a primary path and an alternate path and sends them to a management center. The management center checks whether SRLGs of the two routes overlap, and instructs the node to perform calculation again if the routes overlap. The management center searches for an alternate path having a route overlapping the route of the above described alternate path. When an overlapping alternate path exists, and SRLGs of a primary path corresponding to the alternate path and the above described primary path do not overlap, a link is shared in an overlapping portion of the routes of the two alternate paths.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a communication network, a pathsetting method, and a recording medium having a path setting programrecorded thereon, and more particularly to a communication network, apath setting method, and a recording medium having a path settingprogram recorded thereon in a mesh type communication network.

[0003] 2. Description of the Related Art

[0004] Methods for forming a mesh type network in a public communicationnetwork includes a methods using a cross connecting unit by SynchronousOptical Network (SONET) or Synchronous Digital Hierarchy (SDH)technique, or a method using a cross connecting unit by AsynchronousTransfer Mode (ATM) technique, and various failure recovery schemes areproposed in each method. A failure recovery scheme proposed in T. Wu,“Fiber Network Service Survivability,” Artech House, 1992, Chapter 5 canbe classified into a centralized control scheme and a decentralizedcontrol scheme. The centralized control scheme is a scheme forcontrolling almost all processing steps concerning failure recovery in anetwork or a subnetwork in one centralized control unit, and alarmsindicating failure detection are once centralized in the centralizedcontrol unit, and then the centralized control unit determines aproceeding step to be next performed based on the alarms, and instructsseveral related nodes to perform the proceeding step to detourcommunication traffic from a link or a node where failure occurs. Thedecentralized control scheme is a scheme in which nodes constituting anetwork perform failure recovery processing in an autonomouslydecentralized manner.

[0005] The failure recovery scheme can be also classified into apreplanned scheme and a dynamic scheme. In the preplanned scheme, aroute of an alternate path is previously calculated with respect to aprimary path, and occurrence of failure immediately causes switch to thealternate path. In the dynamic scheme, a route of an alternate path iscalculated after detection of failure, and finding the route causesswitch to the alternate path. The preplanned scheme is classified into a1+1 and 1:1 scheme in which one alternate path is prepared with respectto one primary path, 1:n scheme in which n (an integer not less than 2)primary paths share one alternate path source, and m:n scheme in which nprimary paths share m (an integer not less than 2) alternate pathsources. Sharing the alternate path source achieves an advantage ofincreased use efficiency of the source. However, a conflict may occur insuch a manner that the plurality of primary paths try to take onealternate path source when multiple failures occur, so that caution mustbe taken in determining which primary paths share the alternate pathsource.

[0006] Attention has been recently given to technique of applying animproved protocol that is developed for an Internet Protocol (IP)network to an optical network to achieve high speed provisioning andhigh speed failure recovery of an optical path in the optical network.For example, in Multi-Protocol Label Switching (MPLS) working group ofInternet Engineering Task Force (IETF), standardization of controltechnique of such an optical network is implemented. In an internetdraft: draft-many-ip-optical framework-01.txt submitted to the IETF, aconcept of Shared Risk Link Group (SRLG) is introduced for facilitatingroute calculation of an alternate path in failure recovery. The SRLG isa group consisting of a plurality of links sharing the same physicalsource. Sharing the same physical source means that all the linksbelonging to the SRLG are affected when failure occurs in the sharedphysical source. For example, a plurality of optical fibers in the samepipe are simultaneously affected by one failure of cutting of the pipe.In a wavelength division multiplexed optical network, cutting of oneoptical fiber affects a plurality of wavelengths in the optical fiber.The SRLG is identified by SRLG ID and used in the route calculation ofthe alternate path. For example, in page 24 of draft-many-ip-opticalframework-01.txt, it is described that a primary path and an alternatepath should be adapted not to pass links belonging to the same SRLG in1+1 failure recovery. In page 26 of draft-many-ip-opticalframework-01.txt, it is described that alternate paths corresponding toa plurality of primary paths should be able to share one link simplywhen the plurality of primary paths do not pass links belonging to thesame SRLG.

[0007] In this application, a group of sources sharing a risk such asSRLG is referred to as a risk sharing resource group. Setting paths inconsideration of the risk sharing resource group can prevent a pluralityof paths from being simultaneously affected by one failure to makefailure recovery impossible.

[0008] Examples of path setting methods of this kind are described inNational Publication of International Patent Application No. 11-508421,Japanese Patent Laid-Open No. 9-224.026 and Japanese Patent No. 2770749.

[0009] To calculate the primary path or alternate path in considerationof the risk sharing resource group, it is necessary to know to whichrisk sharing resource group the source such as the link or node in thenetwork belongs. Specifically, in the decentralized control scheme, allnodes must perform route calculation to thereby respectively have risksharing resource group information of the entire network. Withincreasing size of the network, the amount of information becomesenormous to require that each node has a large amount of memory source.

[0010] When a link of alternate paths corresponding to a plurality ofprimary paths are to be shared, in order to determine whether a linkused in calculation of an alternate path corresponding to a certainprimary path can be shared with alternate paths corresponding to otherprimary paths, it is also necessary to know which link other all primarypaths and alternate paths corresponding thereto pass. Path informationis often updated in accordance with set or release of the path, so thatthere is a problem that when all nodes have the pass information of theentire network in the decentralized control scheme, traffic fortransmission of the pass information between the nodes significantlyincreases. This problem also becomes more noticeable with increasingsize of the network.

[0011] One of means for solving the problems is, as described in page 26of draft-many-ip-optical framework-01.txt, to prepare a route serverhaving all necessary information such as topology information of anetwork, information on a risk sharing resource group, or pathinformation, and to perform route calculation of the primary path andalternate path by the centralized control scheme. However, thecentralized control scheme has the following problems: 1) when the sizeof the network is large, load of route calculation centralized in aroute server becomes too large; 2) when failure occurs in the routeserver, route calculation can be no longer performed to cause lowerfailure resistance than the decentralized control scheme.

[0012] Specifically, in the conventional techniques, there are two pathsetting methods:

[0013] (A) a method for imparting all information to the nodes; and

[0014] (B) a method for imparting all information to the centralizedcontrol unit. However, (A) has a problem that traffic for synchronizingthe information between the nodes becomes enormous, and (B) has aproblem that load of calculation is centralized in the centralizedcontrol unit.

SUMMARY OF THE INVENTION

[0015] An object of the present invention is to provide a communicationnetwork, a path setting method, and a recording medium having a pathsetting program recorded thereon capable of preventing load of routecalculation from being centralized in part of units, and capable ofpreventing traffic between nodes from increasing.

[0016] In order to solve the above described problems, a first aspect ofthe present invention provides a communication network including: aplurality of nodes constituting a network; and a management centerconnected to each of the nodes, wherein each of the nodes has topologyinformation of the network, and the management center has information ona risk sharing resource group.

[0017] A second aspect of the present invention provides a managementcenter connected to a plurality of nodes constituting a network, whereinthe management center has information on a risk sharing resource group,and when each of the nodes calculates a first path and a second pathhaving different routes, the management center sends the information onthe risk sharing resource group to the node.

[0018] A third aspect of the present invention provides a plurality ofnodes constituting a network, wherein each of the nodes has topologyinformation of the network, and when calculating a first path and asecond path having different routes, the node obtains information on arisk sharing resource group from a management center connected to thenode.

[0019] A fourth aspect of the present invention provides a path settingmethod in a communication network including a plurality of nodesconstituting a network and a management center connected to each of thenodes, wherein each of the nodes has topology information of thenetwork, and the management center has information on a risk sharingresource group, and the method includes: a first step in which a sourcenode refers to the topology information of the network to calculate aroute of a first path and send the route obtained to the managementcenter; a second step in which the management center refers to theinformation on the risk sharing resource group to return a list of alink group not belonging to the risk sharing resource group that theroute sent from the source node passes to the source node; and a thirdstep in which the source node refers to the list sent from themanagement center to calculate a route of a second path.

[0020] A fifth aspect of the present invention provides a recordingmedium having a path setting program recorded thereon for controlling anode in a communication network including a plurality of nodesconstituting a network and a management center connected to each of thenodes, wherein each of the nodes has topology information of thenetwork, and the management center has information on a risk sharingresource group, and the medium includes: a first step in which a sourcenode refers to the topology information of the network to calculate aroute of a first path and send the route obtained to the managementcenter; a second step in which the management center refers to theinformation on the risk sharing resource group to return a list of alink group not belonging to the risk sharing resource group that theroute sent from the source node passes to the source node, and then thesource node refers to the list sent from the management center tocalculate a route of a second path.

[0021] A sixth aspect of the present invention provides a recordingmedium having a path setting program recorded thereon for a managementcenter in a communication network including a plurality of nodesconstituting a network, and a management center connected to each of thenodes, wherein each of the nodes has topology information of thenetwork, and the management center has information on a risk sharingresource group, and the medium includes a first step in which a sourcenode refers to the topology information of the network to calculate aroute of a first path and send the route obtained to the managementcenter, and then the management center refers to the information on therisk sharing resource group to return a list of a link group notbelonging to the risk sharing resource group that the route sent fromthe source node passes to the source node, and the source node refers tothe list sent from the management center to calculate a route of asecond path.

[0022] According to the first to sixth aspects of the present invention,load of route calculation is decentralized to the nodes and themanagement center to thereby prevent the load of the route calculationfrom being centralized in part of units.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 shows a configuration of a network according to a firstembodiment of the invention;

[0024]FIG. 2 shows a configuration of a node 1;

[0025]FIG. 3 shows a configuration of a management center 2;

[0026]FIG. 4 shows a topology table 12;

[0027]FIG. 5 shows an SRLG table 16;

[0028]FIG. 6 shows a path table 17;

[0029]FIG. 7 shows a port table 19;

[0030]FIG. 8 shows a routing table 18;

[0031]FIG. 9 shows a topology table 12;

[0032]FIG. 10 shows a port table 19;

[0033]FIG. 11 shows a routing table 18;

[0034]FIG. 12 shows a configuration of a node 1;

[0035]FIG. 13 shows a configuration of a management center 2;

[0036]FIG. 14 shows a configuration of a network according to a thirdembodiment of the invention;

[0037]FIG. 15 shows a configuration of a node 1;

[0038]FIG. 16 shows a path table 17;

[0039]FIG. 17 shows a configuration of a management center 2;

[0040]FIG. 18 shows a configuration of a network according to a seventhembodiment of the invention;

[0041]FIG. 19 shows a configuration of a node 1;

[0042]FIG. 20 shows an external routing table 60;

[0043]FIG. 21 shows a topology table 12;

[0044]FIG. 22 shows a configuration of a network according to an eighthembodiment of the invention;

[0045]FIG. 23 shows a configuration of a node 1;

[0046]FIG. 24 is a flowchart showing operation of the first embodimentof the invention;

[0047]FIG. 25 is a flowchart showing operation of the first embodimentof the invention;

[0048]FIG. 26 is a flowchart showing operation of the first embodimentof the invention;

[0049]FIG. 27 is a flowchart showing operation of the first embodimentof the invention;

[0050]FIG. 28 is a flowchart showing operation of a second embodiment ofthe invention;

[0051]FIG. 29 is a flowchart showing operation of the third embodimentof the invention;

[0052]FIG. 30 is a flowchart showing operation of the third embodimentof the invention;

[0053]FIG. 31 is a flowchart showing operation of the third embodimentof the invention;

[0054]FIG. 32 is a flowchart showing operation of a fourth embodiment ofthe invention;

[0055]FIG. 33 is a flowchart showing operation of a fifth embodiment ofthe invention;

[0056]FIG. 34 is a flowchart showing operation of a sixth embodiment ofthe invention;

[0057]FIG. 35 is a flowchart showing operation of the sixth embodimentof the invention;

[0058]FIG. 36 is a flowchart showing operation of the seventh embodimentof the invention;

[0059]FIG. 37 is a flowchart showing operation of the seventh embodimentof the invention;

[0060]FIG. 38 is a flowchart showing operation of the seventh embodimentof the invention;

[0061]FIG. 39 shows a configuration of a node controlled by a pathsetting program; and

[0062]FIG. 40 shows a configuration of a management center controlled bya path setting program.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0063] Now, embodiments of the present invention will be described withreference to the accompanying drawings. A first embodiment will bedescribed first. FIG. 1 shows a configuration of a network of a firstembodiment. With reference to the drawing, six nodes 1 (1-1 to 1-6) arelinked by a two-way link group consisting of four two-way links. In thisembodiment, such a link group is provided by preparing a 4 wavelengthdivision multiplexing transmission line for uplink and downlink,respectively, and a transmission format of the link is STM-16 of SDH. Alink group between the node 1-1 and node 1-3 will be hereinafterreferred to as (1, 3). This network includes a management center 2connected to all nodes 1.

[0064]FIG. 2 shows a configuration of the node 1. With reference to thedrawing, for a link 30 connected to an adjacent node, Section Over Headonly is terminated by a transponder 13 of SDH and converted to a stationinterface 32. The node 1 is also connected to a client (not shown inFIG. 1) by a station interface 32. These station interfaces 32 areswitched by a switch 10. A data communication channel (D1 to D3 bytes)of the Section Over Head of one link 30 in the link group is used as acontrol channel 31, and the control channel 31 is connected to a nodecontrol unit 11. With the control channel 31, the node control unit 11can communicate with a node control unit 11 of an adjacent node. Thenode control unit 11 includes a topology table 12 showing a connectionstate of the entire network, a routing table 18 showing connection ofthe switch 10 in this node, and a port table 19 showing a connectionrelationship between ports of the adjacent node and this node (FIG. 7).The node control unit 11 can communicate with the management center 2via a communication interface 14.

[0065]FIG. 3 shows a configuration of the management center 2. Withreference to the drawing, the management center 2 includes a centralizedcontrol unit 15, which is connected to all nodes 1 via the communicationinterface 14. The centralized control unit 15 stores an SRLG table 16showing a relationship between the link group and SRLG, and a path table17 in which routes and SRLGs of a primary path and an alternate pathcurrently set are recorded.

[0066] Next, operation of the first embodiment will be described withreference to the flowcharts. FIGS. 24 to 27 are flowcharts showing theoperation of the first embodiment. In the flowcharts, at a front of adescription of each step, operation of the node is indicated by (N), andoperation of the management center is indicated by (K).

[0067] First, setting two-way primary path 20-1 and alternate path 21-1whose source node is a node 1-1 and whose destination node is a node 1-5will be considered. Since they are two-way paths, there is no source ordestination in terms of flow of data, but for convenience, a node thatis a source of path setting is set as a source node, and an opposite endis set as a destination node. Seen from a certain node, a source side isreferred to as upstream, and a destination side is referred to asdownstream.

[0068] On this network, in order for each node to understand topology ofthe entire network, for example, a link state routing protocol operatessuch as an extended Open Shortest Path First (OSPF) protocol describedin K. Kompella et al., “OSPF Extensions in Support of MPL (ambda) S,”draft-kompella-ospf-ompls-extensions-00.txt, IETF Internet Draft, July,2000. Therefore, the topology tables 12 of all nodes are synchronized.The topology table 12 at this time is as shown in FIG. 4. Here, the node1-3 is simply indicated as 3. For example, line 1 of FIG. 4 shows thatthe node 1-1 is adjacent to a node 1-2, a metric of a link group (1, 2)connecting both nodes is 1, and the number of currently available linksis 4. The same information is also written in line 4. That is, the node1-2 is adjacent to the node 1-1, the metric of the link group (1, 2)connecting both nodes is 1, and the number of currently available linksis 4. The metric is cost of a link used for route calculation, and thenumber of hops is set as the metric here.

[0069] The node control unit 11 of the node 1-1 refers to the topologytable 12 to calculate the shortest route from the node 1-1 to node 1-5that simply passes link groups having one or more available links, usinga Constrained Shortest Path First (CSPF) algorithm that is a calculationalgorithm of the shortest route under certain constrained conditions(S1). The CSPF algorithm is described, for example, in B. Davie et al.,“MPLS Technology and Applications,” Morgan Kaufmann Publishers, 12000,pages 175 to 180. This calculation provides routes of link groups (1,3), (3, 5) and routes of link groups (1, 4), (4, 5). In this embodiment,when a plurality of routes are obtained, a route having the smallestnode number of the hop next the source node is selected, and the routes(1, 3), (3, 5) are selected here to be the routes of the primary path20-1. Then, the node 1-1 calculates the shortest route from the node 1-1to node 1-5 that simply passes link groups having one or more availablelinks except the link groups (1, 3), (3, 5), also using the CSPFalgorithm. This calculation provides the routes (1, 4), (4, 5) to be theroutes of the alternate path 21-1 (different from the routes shown inFIG. 1) (S1).

[0070] Then, the node 1-1 sends the routes of the obtained primary path20-1 and alternate path 21-1 to the management center 2 (S2). Thecentralized control unit 15 of the management center 2 refers to theSRLG table 16 to check SRLG that the routes of the primary path 20-1 andalternate path 21-1 sent from the node 1-1 pass (S3). Here, the SRLGtable 16 is, for example, as shown in FIG. 5. Line 5 of this drawingshows that the link group (3, 5) belongs to two SRLGs of SRLG 5 and SRLG9. The SRLGs that the primary path 20-1 passes are SRLGs 2, 5, 9, andthe SRLGs that the alternate path 21-1 passes are SRLGs 3, 6, 9, and itis found that SRLG 9 is an overlap between both paths (Y in S4). Thatis, failures may occur simultaneously in the primary path 20-1 andalternate path 21-1. Thus, the management center 2 sends a rejectionmessage with the link group number (4, 5) of overlapping SRLG to thenode controlling unit 11 of the node 1-1 (S5).

[0071] The node 1-1 having received the rejection message calculates theshortest route from the node 1-1 to node 1-5 that passes link groupshaving one or more available links except the link group (4, 5) as wellas the link groups (1, 3), (3, 5) (S6). This provides the routes (1, 4),(4, 6), (5, 6) to be new routes of the alternate path 21-1.

[0072] The node 1-1 again sends the new paths of the primary path 20-1and alternate path 21-1 to the management center 2 (S7). The centralizedcontrol unit 15 of the management center 2 again refers to the SRLGtable 16 (S3) to find that the new alternate path 21-1 passes SRLGs 3,7, 8. These SRLGs do not overlap the SRLGs 2, 5, 9 that the primary path20-1 passes (N in S4, S8).

[0073] The management center 2 searches for an alternate path having aroute overlapping the route of the alternate path 21-1 in the path table17 (S9), but there is no such alternate path at this time (N in S10).Thus, the management center 2 sends a permission message to the node 1-1(S12), and records the routes and SRLGs of the primary path 20-1 andalternate path 21-1 in the path table 17 (S11). A state of the pathtable 17 at this time is shown in FIG. 6.

[0074] The node control unit 11 of the node 1-1 having received thepermission message refers to the port table 19 to set the routing table18 for the primary path 20-1 (S13). The port table 19 of the node 1-1 isas shown in FIG. 7. FIG. 7 shows that a port 0 of the node 1-1 isconnected to a port 0 of the client, a port 1 to a port 1 of the client,ports 2 to 5 to ports 1 to 4 of the node 1-2, ports 6 to 9 to ports 1 to4 of the node 1-3, ports 10 to 13 to ports 1 to 4 of the node 1-4, andall the ports are unused. The node 1-1 is the source node of the primarypath 20-1, and thus it is determined that an upstream node is theclient. Among unused ports connected to the client, the port 0 havingthe smallest number is selected as an upstream port. A downstream nodeof the primary path 20-1 is the node 1-3, and a port 6 having thesmallest number is selected from unused ports connected to the node 1-3as a down stream port. Thus, the routing table 18 of the node 1-1 forthe primary path 20-1 is set as in line 1 of FIG. 8. Then, the node 1-1signals to the node 1-3 via the control channel 31 to indicate that thedownstream port 6 allocated to the primary path 20-1 by the node 1-1 isconnected to the port 1 of the node 1-3. The node 1-3 having received itwrites the upstream node of the primary path 20-1 being the node 1-1 andthe upstream port being the port 1 in the routing table 18. Next, thenode 1-3 selects a downstream port to be connected to the downstreamnode 1-5 in the same manner as the node 1-1 does, and writes it in therouting table 18. The routing table of the node 1-3 for the 20-1 is nowcompleted. Further, the node 1-3 signals to the node 1-5 to indicate thenumber of port of the node 1-5 to which the downstream port allocated tothe primary path 20-1 is connected. The node 1-5 writes the port numberin its own routing table 18 as the upstream port. The node 1-5 is adestination and thus the downstream node is the client, and the node 1-5selects the downstream port connected to the client and also writes itin the routing table 18. In this way, the routing tables 18 of all thenodes on the primary path 20-1 are set. The nodes 1-1, 1-3, 1-5 controlthe switch 10 in accordance with the routing table 18 to open theprimary path 20-1 (S14). As the signaling protocol, extended ResourceReservation Protocol (RSVP) described in D. Saha et al., “RSVPExtensions for Signaling Optical Paths,”draft-saha-rsvp-optical-signaling-00.txt, IETF Internet Draft, 2000 maybe used.

[0075] Then, the node 1-1 sets the routing table 18 for the alternatepath 21-1 as in line 2 of FIG. 8 (S15). Like the primary path 20-1,signaling from the node 1-1 to nodes 1-4, 1-6, and 1-5 successivelywhile selecting a downstream port in each node causes the routing tables18 of all the nodes on the alternate path 21-1 to be set. For thealternate path, each node 1 simply sets the routing table 18 andreserves the port, and does not actually open the path (S16).

[0076] Next, further setting two-way primary path 20-2 and alternatepath 21-2 whose source node is the node 1-1 and whose destination nodeis the node 1-6 will be considered. At this time, a topology table 12 ofthe node 1-1 is as shown in FIG. 9.

[0077] The node 1-1 refers to the topology table 12 (FIG. 9) tocalculate the shortest route from the node 1-1 to node 1-6 that simplypasses link groups having one or more available links, using the CSPFalgorithm. This calculation provides the routes (1, 2), (2, 6), androutes (1, 4), (4, 6), but the routes (1, 2), (2, 6) are selected inaccordance with the above described rule to be the routes of the primarypath 20-2 (S1). Then, the node 1-1 calculates the shortest route fromthe node 1-1 to node 1-6 that simply passes link groups having one ormore available links except the link groups (1, 2), (2, 6), also usingthe CSPF algorithm. This calculation provides the routes (1, 4), (4, 6)to be the routes of the alternate path 21-2 (S1).

[0078] Then, the node 1-1 sends the routes of the obtained primary path20-2 and alternate path 21-2 to the management center 2 (S2). Themanagement center 2 refers to the SRLG table 16 (FIG. 5) (S3) to findthat the SRLGs that the primary path 20-2 passes are SRLGs 1, 4, and theSRLGs that the alternate path 21-2 passes are SRLGs 3, 7, and theseSRLGs do not overlap (N in S4, S8).

[0079] Subsequently, the management center 2 searches for an alternatepath having a route overlapping the route of the alternate path 21-2 inthe path table 17 (FIG. 6) (S9). This time, the primary path 20-1 andalternate path 21-1 have been recorded in the path table 17 of themanagement center 2, and the route of the alternate path 21-1 overlapthe route of the alternate path 21-1 in the link groups (1, 4), (4, 6)(Y in S10).

[0080] In descriptions in the flowcharts, the primary path and alternatepath currently set are indicated as the primary path 1 and alternatepath 1, and the primary path and alternate path already set areindicated as the primary path 2 and alternate path 2.

[0081] The management center 2 compares the SRLGs of the primary path20-2 and primary path 20-1 corresponding to the alternate path 21-1(S17) to find that the SRLGs of both of them do not overlap (N in S18).

[0082] As a result, the management center 2 sends a permission messageto the node 1-1, with the number of the alternate path 21-1 and thenumbers of the overlapping link groups (1, 4), (4, 6) added (S20). Thisis for setting the alternate path 21-2 so as to share links with thealternate path 21-1 in the link groups (1, 4), (4, 6). Simultaneously,the management center 2 records the routes and SRLGs of the primary path20-2 and alternate path 21-2 in the path table 17 (Sl9).

[0083] The node 1-1 having received the permission message with thenumber of the alternate path 21-1 and the numbers of the link groups (1,4), (4, 6) first refers to the port table 19 to set the routing table 18for the primary path 20-2 (S21). The port table 19 at this time is asshown in FIG. 10. It is determined that an upstream node of the primarypath 20-2 is the client, an upstream port is a port 1, a downstream portis the node 1-2, and a downstream port is a port 2, and the node 1-1writes them in the routing table 18. Then, the node 1-1 signals to thenode 1-2 to indicate the port number 1 of the downstream node 1-2connected to the downstream port 2. The node 1-2 selects the port 1 asthe upstream port, selects the downstream port by itself, writes them inthe routing table 18, and signals to the node 1-6 that is the downstreamnode to indicate the upper port number. The node 1-6 is a destinationnode and thus selects the downstream port from the ports connected tothe client and writes it in the routing table. In this way, the routingtables 18 of all the nodes on the primary path 20-2 are set, and inaccordance therewith, each node controls the switch 10 to open theprimary path 20-2.

[0084] Then, the node 1-1 sets the routing table 18 for the alternatepath 21-2 (S23). The upstream node is the client, the upstream port isthe port 1, and the downstream node is the node 1-4. When selecting thedownstream port, the node 1-1 is instructed by the management center 2to have the alternate path 21-1 and alternate path 21-2 share the linkin the link group (1, 4) and thus selects the port 10 identical to thedownstream port of the alternate path 21-1 as the downstream port of thealternate path 21-2. Therefore, the routing table 18 is set as shown inFIG. 11 (S23). Then, the node 1-1 signals to the node 1-4 to indicatethe upstream port number 1. Simultaneously, the node 1-1 instructs thenode 1-4 to select the same port as allocated to the alternate path21-1, as the downstream port to be allocated to the alternate path 21-2.The extended RSVP described above has no such function, but the RSVP caneasily extend its function by adding an object. A function ofinstructing a certain node to allocate the port having allocated to acertain path to another path may be added by adding a new object.Thereafter, similarly signaling from the node 1-4 to node 1-6 causes therouting tables 18 of all the nodes on the alternate path 21-2 to be set(S24).

[0085] As described above, two pairs of primary paths and alternatepaths respectively having no overlapping SRLG can be set, and twoalternate paths can be set to share a link on a certain link group,thereby achieving effective use of source.

[0086] In the above described embodiment, the SRLGs of the primary path20-1 and alternate path 21-1 do not overlap, but the case where theSRLGs overlap will be now described with reference to FIGS. 26 and 27.When the SRLGs of the primary paths overlap (Y in S18), the respectivepaths and SRLGs of the primary path 20-1 and alternate path 21-1 are setin the path table (S25).

[0087] Then, the management center sends a permission message to thenode, but do not add information on the number of the alternate path21-1 or the number of overlapping link group at this time (S26).

[0088] The node 1-1 having received the permission message first refersto the port table 19 and sets the routing table 18 for the primary path20-2 (S27). Then, the node 1-1 signals to the downstream nodessuccessively. Thus, the routing tables 18 of all the nodes on theprimary path 20-2 are set, and in according therewith, each nodecontrols the switch 10 to open the primary path 20-2 (S28). A series ofsteps of opening the primary path 20-2 is the same as in the abovedescribed example.

[0089] Then, the node 1-1 sets the routing table 18 for the alternatepath 21-2 (S29). When selecting the downstream port, the node 1-1 is notinstructed by the management center 2 to have the alternate path 21-1and alternate path 21-2 share the link on the overlapping link group,and thus selects a port different from the downstream port of thealternate path 21-1 as the downstream port of the alternate path 21-2.Then, the node 1-1 signals to the node 1-4 to indicate the upstream portnumber selected in such a manner that the links do not overlap.Thereafter, similarly signaling from the node 1-4 to node 1-6 causes therouting tables 18 of all the nodes on the alternate path 21-2 to be set(S30).

[0090] As described above, two pairs of primary paths and alternatepaths respectively having overlapping SRLGs can be set, and twoalternate paths can be set to have no overlapping link on a certain linkgroup. In this way, even when the SRLGs of the primary paths overlap andsimultaneous disconnection may occur, the routes are set in such amanner that links of the alternate paths do not overlap, therebypreventing fatal failure.

[0091] Next, a second embodiment will be described. FIG. 1 is also usedin the second embodiment. Configurations of a node 1 and a managementcenter 2 in the second embodiment are shown in FIG. 12 and FIG. 13,respectively. In the second embodiment, a node control unit 11 of thenode 1 includes a topology table 12, SRLG table 16, routing table 18,and port table 19, and a centralized control unit 15 of the managementcenter 2 includes a path table 17. The other configurations areidentical to those in the first embodiment.

[0092] Now, operation of the second embodiment will be described withreference to the flowchart. FIG. 28 is a flowchart showing the operationof the second embodiment. First, setting two-way primary path 20-1 andalternate path 21-1 whose source node is a node 1-1 and whosedestination node is a node 1-5 will be considered.

[0093] The node 1-1 refers to the topology table 12 and SRLG table 16 tocalculate the shortest route and the second shortest route from the node1-1 to node 1-5 that simply pass link groups having one or moreavailable links in such a manner that both routes do not pass the sameSRLG, as routes of the primary path 20-1 and the alternate path 21-1(S31). An algorithm calculating such a pair of routes is described, forexample, in J. Suurballe, “Disjoint Paths in a Network,” Networks, vol.4, 1974. This calculation provides routes (1, 3), (3, 5) as the routesof the primary path 20-1 and routes (1, 4), (4, 6), (5, 6) as the routesof the alternate path 21-1.

[0094] The node 1-1 sends information on the routes and passing SRLGs ofthe primary path 20-1 and alternate path 21-1 to the management center 2(S32). The management center 2 refers to the path table 17 to search foran alternate path having a route overlapping the route of the alternatepath 21-1 (S33), but there is no such alternate path (N in S34). Thus,the management center 2 sends a permission message to the node 1-1(S35), and records the routes of the primary path 20-1 and alternatepath 21-1 in the path table 17 (S36). Therefore, a state of the pathtable 17 is as shown in FIG. 6.

[0095] The node 1-1 having received the permission message signals tonodes 1-3, 1-5 (S14) as in the first embodiment, and thus the routingtables 18 for the primary path 20-1 are set in all the nodes 1 on theprimary path 20-1 (S13). The nodes 1-1, 1-3, 1-5 control an opticalswitch 10 in accordance with the routing table 18 to open the primarypath 20-1. Then, the node 1-1 signals to the nodes 1-4, 1-6, 1-5 (S16),and thus the routing tables 18 for the alternate path 21-1 are set inall the nodes 1 on the alternate path 21-1 (S15).

[0096] Next, further setting two-way primary path 20-2 and alternatepath 21-2 whose source node is the node 1-1 and whose destination nodeis the node 1-6 will be considered.

[0097] The node 1-1 refers to the topology table 12 (FIG. 9) and theSRLG table 16 (FIG. 5) to calculate the shortest route and the secondshortest route from the node 1-1 to node 1-6 that simply pass linkgroups having one or more available links in such a manner that bothroutes do not pass the same SRLG, as routes of the primary path 20-2 andthe alternate path 21-2 (S31). This calculation provides routes (1, 2),(2, 6) as the routes of the primary path 20-2 and routes (1, 4), (4, 6)as the routes of the alternate path 21-2.

[0098] Then, the node 1-1 sends the routes obtained of the primary path20-2 and alternate path 21-2 to the management center 2 (S32). Themanagement center 2 refers to the path table 17 (FIG. 6) to search foran alternate path having a route overlapping the route of the alternatepath 21-2 (S33). This time, the primary path 20-1 and alternate path21-1 have been recorded in the path table 17, and the routes of thealternate path 21-1 overlaps the routes of the alternate path 21-2 inthe link groups (1, 4), (4, 6) (Y in S34). The management center 2compares the SRLGs of the primary path 20-2 and primary path 20-1corresponding to the alternate path 21-1 (S17) to find that the SRLGs ofboth of them do not overlap (N in S18).

[0099] As a result, the management center 2 sends a permission messageto the node 1-1, with the number of the alternate path 21-1 and thenumbers of the overlapping link groups (1, 4), (4, 6) added (S20).Simultaneously, the management center 2 records the routes and SRLGs ofthe primary path 20-2 and alternate path 21-2 in the path table 17(S19).

[0100] Thereafter, in accordance with completely the same steps as inthe first embodiment, the primary path 20-2 and alternate path 21-2 areset, and the alternate paths 21-1 and 21-2 share links on the linkgroups (1, 4), (4, 6).

[0101] As described above, two pairs of primary paths and alternatepaths respectively having no overlapping SRLG can be set, and twoalternate paths can be set to share a link on a certain link group.Processing when the SRLGs of the primary path 1 and primary path 2overlap is the same as in the first embodiment.

[0102] Next, a third embodiment will be described. FIG. 14 shows aconfiguration of a network of the third embodiment. In the thirdembodiment, there is no centralized management center. FIG. 15 shows aconfiguration of a node 1. The node 1 in this embodiment additionallyhas a path table 17. However, the path table 17 simply recordsinformation on a primary path passing the node 1 itself and an alternatepath corresponding thereto, or an alternate path passing the node 1itself and a primary path corresponding thereto. There is nocommunication interface for communication with the centralizedmanagement center. The other configurations are identical to those inthe second embodiment.

[0103] Now, operation of the third embodiment will be described withreference to the flowchart. FIGS. 29 to 31 are flowcharts showing theoperation of the third embodiment. First, setting two-way primary path20-1 and alternate path 21-1 whose source node is a node 1-1 and whosedestination node is a node 1-5 will be considered.

[0104] The node 1-1 calculates routes of the primary path 20-1 andalternate path 21-1 in the same manner as in the second embodiment, andobtains routes (1, 3), (3, 5) as the routes of the primary path 20-1 androutes (1, 4), (4, 6), (5, 6) as the routes of the alternate path 21-1(S51). The node 1-1 records information on the routes and SRLGs of theprimary path 20-1 and alternate path 21-1 in the path table 17 as shownin FIG. 6 (S52).

[0105] Next, the node 1-1 sets the routing table 18 for the primary path20-1 (S53). The setting manner is the same as in the first embodiment.That is, the routing table 18 is set as in line 1 of FIG. 11.

[0106] Then, the node 1-1 signals to a downstream node 1-3 to indicatean upstream port number 1 (S54). The node 1-3 writes the number in itsown routing table 18. This signaling also indicates the information onthe routes and SRLGs of the primary path 20-1 and alternate path 21-1,and the node 1-3 records the information in its own path table 17.However, it is sufficient for the information to include that of a routeof the althernate path 21-1 and SRLGs of the primary path 20-1, atleast.

[0107] Next, the node 1-3 sets the routing table 18 for the primary path20-1. The setting manner is the same as in the first embodiment.

[0108] Then, the node 1-3 signals to the downstream node 1-5 to indicatean upstream port number. The node 1-5 writes the number in its ownrouting table 18. This signaling also indicates the information on theroutes and SRLGs of the primary path 20-1 and alternate path 21-1, andthe node 1-5 records the information in its own path table 17.

[0109] Next, the node 1-5 sets the routing table 18 for the primary path20-1. The setting manner is the same as in the first embodiment.

[0110] In this way, setting of the routing tables 18 for the primarypath 20-1 in the nodes on the primary path 20-1 is completed, and eachnode controls a switch 10 in accordance therewith to open the primarypath 20-1.

[0111] Next, the node 1-1 sets the routing table 18 for the alternatepath 21-1 (S55). The node 1-1 refers to the path table 17 and searchesfor another alternate path passing the link group (1, 4) like thealternate path 21-1 (S56), but there is no such alternate path (N inS57). Thus, the node 1-1 allocates an unused port to a downstream portof the alternate path 21-1 (S58). That is, the routing table 18 is setas in line 2 of FIG. 11.

[0112] Then, the node 1-1 signals to a downstream node 1-4 of thealternate path 21-1 to indicate an upstream port number 1 (S59). Thenode 1-4 writes the number in its own routing table 18 (N in S60, S61).This signaling also indicates the information on the routes and SRLGs ofthe primary path 20-1 and alternate path 21-1.

[0113] The node 1-4 refers to its own path table 17 and searches foranother alternate path passing the link group (4, 6) like the alternatepath 21-1 (S56), but there is no such alternate path (N in S57). Thus,the node 1-4 allocates an unused port to a downstream port of thealternate path 21-1 (S58). The node 1-4 records the information on theroutes and SRLGs of the primary path 20-1 and alternate path 21-1 in thepath table 17.

[0114] Then, the node 1-4 signals to a downstream node 1-6 of thealternate path 21-1 to indicate an upstream port number (S59). The node1-6 writes the number in its own routing table 18 (N in S60, S61). Thissignaling also indicates the information on the routes and SRLGs of theprimary path 20-1 and alternate path 21-1.

[0115] The node 1-6 refers to its own path table 17 and searches foranother alternate path passing the link group (5, 6) like the alternatepath 21-1 (S56), but there is no such alternate path (N in S57). Thus,the node 1-6 allocates an unused port to a downstream port of thealternate path 21-1 (S58). The node 1-6 records the information on theroutes and SRLGs of the primary path 20-1 and alternate path 21-1 in thepath table 17.

[0116] Next, the node 1-6 signals to a downstream node 1-5 of thealternate path 21-1 to indicate an upstream port number (S59). The node1-5 writes the number in its own routing table 18 (S60) This signalingalso indicates the information on the routes and SRLGs of the primarypath 20-1 and alternate path 21-1.

[0117] The node 1-5 is a destination node of the alternate path 21-1 (Yin S61), and thus allocates the same port as allocated to the primarypath 20-1, that is, the port connected to the client, to the downstreamport of the alternate path 21-1 (S62). The node 1-5 also records theinformation on the routes and SRLGs of the primary path 20-1 andalternate path 21-1 in the path table 17.

[0118] In this way, setting of the routing tables 18 for the alternatepath 21-1 in the nodes on the alternate path 21-1 is completed (S63).

[0119] Next, further setting two-way primary path 20-2 and alternatepath 21-2 whose source node is the node 1-1 and whose destination nodeis the node 1-6 will be considered.

[0120] The node 1-1 calculates routes of the primary path 20-2 andalternate path 21-2 in the same manner as in the second embodiment, andobtains routes (1, 2), (2, 6) as the routes of the primary path 20-2 androutes (1, 4), (4, 6) as the routes of the alternate path 21-1 (S51).The node 1-1 records information on the routes and SRLGs of the primarypath 20-2 and alternate path 21-2 in the path table 17 as shown in FIG.16 (S52).

[0121] Next, the node 1-1 sets the routing table 18 for the primary path20-2 (S53). The setting manner is the same as in the first embodiment.That is, the routing table 18 is set as in line 3 of FIG. 11.

[0122] Then, the node 1-1 signals to a downstream node 1-2 of theprimary path 20-2 to indicate an upstream port number 1 (S54). The node1-2 writes the number in its own routing table 18. This signaling alsoindicates the information on the routes and SRLGs of the primary path20-2 and alternate path 21-2, and the node 1-2 records the informationin its own path table 17.

[0123] Next, the node 1-2 sets the routing table 18 for the primary path20-2 (S53). The setting manner is the same as in the first embodiment.

[0124] Then, the node 1-2 signals to the downstream node 1-6 to indicatean upstream port number. The node 1-6 writes the number in its ownrouting table 18. This signaling also indicates the information on theroutes and SRLGs of the primary path 20-2 and alternate path 21-2, andthe node 1-6 records the information in its own path table 17.

[0125] Next, the node 1-6 sets the routing table 18 for the primary path20-2. The setting manner is the same as in the first embodiment.

[0126] In this way, setting of the routing tables 18 for the primarypath 20-2 in the nodes on the primary path 20-2 is completed, and eachnode controls a switch 10 in accordance therewith to open the primarypath 20-2.

[0127] Next, the node 1-1 sets the routing table 18 for the alternatepath 21-2 (S55). The node 1-1 refers to the path table 17 (FIG. 16) andsearches for another alternate path passing the link group (1, 4) likethe alternate path 21-2 (S56). The alternate path 21-1 applies theretohere (Y in S57). The node 1-1 compares the SRLGs of the primary path20-1 and primary path 20-2 that are recorded in the path table 17 (S68)to find that the SRLGs of both of them do not overlap (N in S69). Thus,the node 1-1 allocates the same port 10 as allocated to the alternatepath 21-1, as the downstream port of the alternate path 21-2 (S70).Therefore, the routing table of the node 1-1 is as shown in FIG. 11.

[0128] Then, the node 1-1 signals to the downstream node 1-4 of thealternate path 21-2 to indicate an upstream port number 1 (S71). Thenode 1-4 writes the number in its own routing table 18 (Nin S72, S73).This signaling also indicates the information on the routes and SRLGs ofthe primary path 20-2 and alternate path 21-2, and the node 1-4 recordsthe information in its own path table 17.

[0129] The node 1-4 refers to the path table 17 and searches for anotheralternate path passing a link group (4, 6) like the alternate path 21-2(S56). The alternate path 21-1 applies thereto here (Y in S57). Thus,the node 1-4 compares the SRLGs of the primary path 20-1 and primarypath 20-2 that are recorded in the path table 17 (S68) to find that theSRLGs of both of them do not overlap (N in S69). Therefore, the node 1-4allocates the same port as allocated to the alternate path 21-1, as thedownstream port of the alternate path 21-2 (S70).

[0130] Next, the node 1-4 signals to the downstream node 1-6 of thealternate path 21-2 to indicate an upstream port number (S71). The node1-6 writes the number in its own routing table 18 (S72). This signalingindicates the information on the routes and SRLGs of the primary path20-2 and alternate path 21-2.

[0131] The node 1-6 is a destination node of the alternate path 21-2 (Yin S73), and thus allocates the same port as allocated to the primarypath 20-2, that is, the port connected to the client, to the downstreamport of the alternate path 21-2 (S74). The node 1-6 also records theinformation on the routes and SRLGs of the primary path 20-2 andalternate path 21-2 in the path table 17. In this way, setting of therouting tables 18 for the alternate path 21-2 in the nodes on thealternate path 21-2 is completed (S75). The alternate paths 21-1 and21-2 share links on the link groups (1, 4), (4, 6).

[0132] When the SRLGs of the primary path 20-1 and primary path 20-2overlaps at step 69 (Y in S69), the node 1-4 allocates a port differentfrom that allocated to the alternate path 21-1, as the downstream portof the alternate path 21-2 (S76).

[0133] As described above, two pairs of primary paths and alternatepaths respectively having no overlapping SRLG can be set, and twoalternate paths can be set to share a link on a certain link group.

[0134] Next, a fourth embodiment will be described. In the fourthembodiment, configurations of a network, nodes, and a management centerare the same as in the first embodiment, and calculation steps only ofroutes of a primary path and an alternate path are different.

[0135] Now, operation of the fourth embodiment will be described withreference to the flowchart. FIG. 32 is a flowchart showing the operationof the fourth embodiment. A node control unit 11 of a node 1-1 firstcalculates a route of a primary path 20-1 in the same manner as in thefirst embodiment (S81). Then, the node 1-1 sends the route obtained ofthe primary path 20-1 to a management center 2 (S82). A centralizedcontrol unit 15 of the management center 2 records the route in a pathtable 17. Next, the centralized control unit 15 refers to a SRLG table16 to prepare a list of a link group not belonging to SRLG that theroute of the primary path 20-1 sent from the node 1-1 passes, andreturns the list to the node 1-1 (S83). Here, a list of link groups (1,2), (1, 4), (2, 6), (4, 6), (5, 6) is sent. The node 1-1 calculates theshortest route from the node 1-1 to node 1-5 simply using link groupshaving one or more available links among link groups included in thelist (S84). This provides the routes (1, 4), (4, 6), (5, 6) to be theroutes of the alternate path 21-1.

[0136] The node 1-1 sends the routes of the alternate path 21-1 to themanagement center 2 (S85). The management center 2 searches for analternate path having a route overlapping the route of the alternatepath 21-1 in the path table 17 (S86), but there is no such alternatepath at this time (N in S87). Thus, the management center 2 sends amessage indicating “share no resource” to the node 1-1 (S88), andrecords the routes of the alternate path 21-1 in the path table 17. Astate of the path table 17 at this time is as shown in FIG. 6.

[0137] Subsequently, the node 1-1 signals to the nodes on the primarypath 20-1 and alternate path 21-1 as in the first embodiment, and setsthe primary path 20-1 and alternate path 21-1 (S89).

[0138] Next, further setting two-way primary path 20-2 and alternatepath 21-2 whose source node is the node 1-1 and whose destination nodeis the node 1-6 in this network will be considered.

[0139] The node 1-1 calculates a route of a primary path 20-2 also inthe same manner as in the first embodiment (S81). The node 1-1 sends theroute to the management center 2 (S82). The centralized control unit 15of the management center 2 records the route in the path table 17. Next,the centralized control unit 15 refers to the SRLG table 16 to prepare alist of a link group not belonging to SRLG that the route of the primarypath 20-2 sent from the node 1-1 passes, and returns the list to thenode 1-1 (S83). Here, a list of link groups (1, 3), (1, 4), (3, 5), (4,5), (4, 6), (5, 6) is sent. The node 1-1 calculates the shortest routefrom the node 1-1 to node 1-5 simply using link groups having one ormore available links among link groups included in the list (S84). Thisprovides the routes (1, 4), (4, 6) to be the routes of the alternatepath 21-2.

[0140] The node 1-1 sends the routes of the alternate path 21-2 to themanagement center 2 (S85), and the centralized control unit 15 of themanagement center 2 records the routes in the path table 17.

[0141] Then, the management center 2 searches for an alternate pathhaving a route overlapping the route of the alternate path 21-2 in thepath table 17 (S86), and determines whether a resource of the alternatepath can be shared based on SRLG of a corresponding primary path (S87).Operation thereafter is completely the same as in the first embodiment.As described above, two pairs of primary paths and alternate pathsrespectively having no overlapping SRLG can be set, and two alternatepaths can be set to share a link on a certain link group.

[0142] Next, a fifth embodiment will be described. In the fifthembodiment, configurations of a network and nodes are the same as in thefirst embodiment, and a configuration of a management center isdifferent. The fifth embodiment is also similar to the fourth embodimentin that calculation results of routes of a primary path are sent to themanagement center before routes of an alternate path are calculated.

[0143] The management center 2 in this embodiment includes an SRLG table16 as shown in FIG. 17, but includes no path table 17. In thisembodiment, setting manner of a primary path 20-1 and alternate path21-1 will be described.

[0144] Now, operation of the fifth embodiment will be described withreference to the flowchart. FIG. 33 is a flowchart showing the operationof the fifth embodiment. A node control unit 11 of a node 1-1 firstcalculates a route of a primary path 20-1 in the same manner as in thefourth embodiment (S81 in FIG. 32). Then, the node 1-1 sends the routeobtained of the primary path 20-1 to a management center 2 (S82). Acentralized control unit 15 of the management center 2 refers to a SRLGtable 16 to prepare a list of a link group not belonging to SRLG thatthe route of the primary path 20-1 sent from the node 1-1 passes, andreturns the list to the node 1-1 (S83). Here, a list of link groups (1,2), (1, 4), (2, 6), (4, 6), (5, 6) is sent. The node 1-1 calculates theshortest route from the node 1-1 to node 1-5 simply using link groupshaving one or more available links among link groups included in thelist (S84). This provides the routes (1, 4), (4, 6), (5, 6) to be theroute of the alternate path 21-1.

[0145] Subsequently, the node 1-1 signals to the nodes on the primarypath 20-1 and alternate path 21-1 as in the first embodiment, and setsthe primary path 20-1 and alternate path 21-1 (S91 in FIG. 33).

[0146] In this way, a pair of primary path and alternate path having nooverlapping SRLG can be set.

[0147] Next, a sixth embodiment will be described. In the sixthembodiment, configurations of a network and nodes are the same as in thefirst embodiment, and a configuration of a management center isdifferent. A management center 2 in this embodiment includes a pathtable 17 as shown in FIG. 13, but includes no SRLG table 16.

[0148] Now, operation of the sixth embodiment will be described withreference to the flowcharts. FIGS. 34 and 35 are flowcharts showing theoperation of the sixth embodiment. A node control unit 11 of a node 1-1first calculates a route of a primary path 20-1 in the same manner as inthe first embodiment. This provides the routes (1, 3), (3, 5) of theprimary path 20-1.

[0149] The node 1-1 calculates the shortest route from the node 1-1 tonode 1-5 that simply passes link groups having one or more availablelinks except the link groups (1, 3), (3, 5). This calculation providesthe routes (1, 4), (4, 5) to be the routes of the alternate path 21-1(S101).

[0150] Then, the node 1-1 sends the routes of the obtained primary path20-1 and alternate path 21-1 to the management center 2 (S102). Themanagement center 2 searches for an alternate path having a routeoverlapping the route of the alternate path 21-1 in the path table 17(S103), but there is no such alternate path at this time (N in S104).Thus, the management center 2 sends a message indicating “have no linkshared in setting the alternate path 21-2” to the node 1-1 (S105), andrecords the routes of the primary path 20-1 and alternate path 21-1 inthe path table 17.

[0151] Subsequently, the node 1-1 signals to the nodes on the primarypath 20-1 and alternate path 21-1 as in the first embodiment, and setsthe primary path 20-1 and alternate path 21-1 (S106).

[0152] Next, further setting two-way primary path 20-2 and alternatepath 21-2 whose source node is the node 1-1 and whose destination nodeis the node 1-6 in this network will be considered.

[0153] The node 1-1 calculates a route of a primary path 20-2 also inthe same manner as in the first embodiment (S81). This provides theroutes (1, 2), (2, 6) of the primary path 20-2.

[0154] Then, the node 1-1 calculates the shortest route from the node1-1 to node 1-6 that simply passes link groups having one or moreavailable links except the link groups (1, 2), (2, 6). This calculationprovides the routes (1, 4), (4, 6) to be the routes of the alternatepath 21-2 (S101).

[0155] Next, the node 1-1 sends the routes of the obtained primary path20-2 and alternate path 21-2 to the management center 2 (S102). Themanagement center 2 searches for an alternate path having a routeoverlapping the route of the alternate path 21-2 in the path table 17(S103). The alternate path 21-1 overlaps the alternate path 21-2 in thelink group (1, 4) (Y in S104). Thus, the management center 2 sends amessage indicating “have the link shared with the alternate paths 21-1in the link group (1, 4) in setting the alternate path 21-2” to the node1-1 (S107), and records the route of the primary path 20-2 and alternatepath 21-2 in the path table 17.

[0156] Subsequently, the node 1-1 signals to the nodes on the primarypath 20-2 and alternate path 21-2 as in the first embodiment, and setsthe primary path 20-2 and alternate path 21-2 (S108).

[0157] As described above, two pairs of primary paths and alternatepaths respectively sharing no link can be set, and two alternate pathscan be set to share a link on a certain link group.

[0158] Next, a seventh embodiment will be described. A configuration ofa network of the seventh embodiment is shown in FIG. 18. In thisembodiment, the network consists of three subnetworks 3-a, 3-b, 3-c. Aconfiguration of each subnetwork is the same as that of the network inthe first embodiment. A node 1-5 of the subnetwork 3-a and a node 1-2 bof the subnetwork 3-b are connected by a primary link group 40-ab and analternate link group 41-ab respectively consisting of four links. Usingthe two link groups, failure recovery by an Automatic ProtectionSwitching (APS) scheme is performed between the nodes 1-5 a and 1-2 b.The APS scheme is described in T. Wu, “Fiber Network ServiceSurvivability,” Artech House, 1992, Chapter 3 and soon. Likewise,failure recovery by the APS scheme using a primary link group 40-ac andalternate link group 41-ac is performed between the subnetworks 3-a and3-c.

[0159] A configuration of a node 1 in this embodiment is shown in FIG.19. The configuration is the same as that of the node 1 in the firstembodiment except for presence of an external routing table 60 in a nodecontrol unit 11. A topology table 12 of the node 1 in the subnetwork 3-aholds topology information in the subnetwork 3-a, that is, the contentsof FIGS. 4 and 21. The external routing table 60 shows a node that aroute passes when a path to a destination node in another subnetwork isset, that is, a boundary node. An example of the external routing table60 is shown in FIG. 20. FIG. 20 shows that the node 1-5 a is a boundarynode when a path whose destination is the subnetwork 3-b is set, andthat the node 1-6 a is a boundary node when a path whose destination isthe subnetwork 3-c. A configuration of a management center 2 of thisembodiment is the same as that of the management center 2 in the firstembodiment. An SRLG table 16 in the management center 2 of thesubnetwork 3-a holds SRLG information in the subnetwork 3-a, that is,the same contents as FIG. 5. In a path table 17, routes and SRLGs of thepaths in the subnetwork 3-a are recorded.

[0160] Now, operation of the seventh embodiment will be described withreference to the flowcharts. FIGS. 36 and 37 are flowcharts showing theoperation of the seventh embodiment.

[0161] Setting a primary path 20-1 and alternate path 21-1 whose sourceis the node 1-1 a in the subnetwork 3-a and whose destination is thenode 1-5 b in the subnetwork 3-b will be considered.

[0162] A node control unit 11 of the node 1-1 a first refers to theexternal routing table 60 to find that the route may pass the node 1-5 awhen the path whose destination is the node in the subnetwork 3-b (S111)is set. The node control unit 11 of the node 1-1 a refers to thetopology table 12 to calculate the shortest route from the node 1-1 a tonode 1-5 a that simply passes link groups having one or more availablelinks, using the CSPF algorithm. This provides the routes (1, 3), (3, 5)of the primary path 20-1 (S81 in FIG. 32).

[0163] Then, the node 1-1 a sends the routes obtained (1, 3), (3, 5) ofthe primary path 20-1 to the management center 2-a (S82). A centralizedcontrol unit 15 of the management center 2-a records the routes in thepath table 17. Next, the centralized control unit 15 refers to the SRLGtable 16 to prepare a list of a link group not belonging to SRLG thatthe routes of the primary path 20-1 sent from the node 1-1 a pass, andreturns the list to the node 1-1 a (S83). Here, a list of link groups(1, 2), (1, 4), (2, 6), (4, 6), (5, 6) is sent. The node 1-1 acalculates the shortest route from the node 1-1 a to node 1-5 a simplyusing link groups having one or more available links among link groupsincluded in the list (S84). This provides the routes (1, 4), (4, 6), (5,6) to be the routes of the alternate path 21-1.

[0164] The node 1-1 a also sends the routes of the alternate path 21-1to the management center 2-a (S85). The management center 2-a searchesfor an alternate path having a route overlapping the route of thealternate path 21-1 in the path table 17 (S86), but there is no suchalternate path at this time (N in S87). Thus, the management center 2-asends a message indicating “share no resource” to the node 1-1 a (S88),and records the routes of the alternate path 21-1 in the path table 17.A state of the path table 17 at this time is shown in FIG. 6.

[0165] Then, the node 1-1 a refers to a port table 19 to set the routingtable 18 for the primary path 20-1 (S113 in FIG. 36). The node 1-1 a isthe source node of the primary path 20-1, and thus an upstream node is aclient. Among ports connected to the client, the port 0 having thesmallest number is selected as an upstream port. A downstream node ofthe primary path 20-1 is the node 1-3 a, and a port 6 having thesmallest number is selected from unused ports connected to the node 1-3as a downstream port. Thus, the routing table 18 of the node 1-1 a forthe primary path 20-1 is set as in line 1 of FIG. 8.

[0166] Then, the node 1-1 a sends a setting request message of theprimary path 20-1 that is a kind of signaling messages to the node 1-3 avia a control channel 31. The setting request message includesinformation such as identification data of the message, identificationdata indicating that this path is the primary path, path number, sourcenode number, destination node number, route information to the node 1-5a, and upstream port number of the downstream node. The upstream portnumber of the downstream node is a port of the node 1-3 a connected to adownstream port 6 allocated to the primary path 20-1 by the node 1-1 a,that is, a port 1.

[0167] The node 1-3 a having received the setting request message writesthe upstream node of the primary path 20-1 being the node 1-1 a and theupstream port being the port 1 in the routing table 18. Next, the node1-3 a selects a downstream port to be connected to the downstream node1-5 a in the same manner as the node 1-1 a does, and writes it in therouting table 18. The routing table of the node 1-3 a for the 20-1 isnow completed. Further, the node 1-3 a rewrites the upstream port numberof the downstream node in the setting request message, and sends thesetting request message to the node 1-5 a.

[0168] The node 1-5 a can find the upstream node (node 1-3 a) and theupstream port number for the primary path 20-1 by the setting requestmessage, and thus writes them in its own routing table 18. The node 1-5a has known that the node 1-5 a itself is the boundary node for the pathto the subnetwork 3-b. In the setting request message of the primarypath 20-1 received by the node 1-5 a, it is also written that thedestination of the path is the node 1-5 b in the subnetwork 3-b, so thatthe node 1-5 a writes the node 1-2 b in the routing table 18 as adownstream node number for the primary path 20-1, and selects the porthaving the smallest port number from the ports connected to the linkincluded in the link group 40-ab as the downstream port, and also writesit in the routing table 18. Then, the node 1-5 a rewrites the upstreamport number of the downstream node in the setting request message intothe port number of the node 1-2 b connected to the downstream portselected by the node 1-5 a itself, and sends the setting request messageto the node 1-2 b (S114).

[0169] Next, the node 1-1 a sets the routing table 18 for the alternatepath 21-1 as in line 2 of FIG. 8 (S115). The node 1-1 a is the source ofthe alternate path 21-1, so that the upstream node and upstream port areidentical to those of the primary path 20-1, the downstream node is anode 1-4 a, and a port 10 having the smallest port number is selectedfrom the ports connected to the node 1-4 a as the downstream port.

[0170] Subsequently, the node 1-1 a generates a setting request messageof the alternate path 21-1 and sends it to the node 1-4 a via thecontrol channel 31. The setting request message includes informationsuch as identification data of the message, identification dataindicating that this path is the alternate path, path number, sourcenode number, destination node number, route information to the node 1-5a, upstream port number of the downstream port, and identification dataindicating that this path shares no resource with another alternatepath. The upstream port number of the downstream node is set to 10 thatis the port number of the node 1-4 a connected to the port 10 of thenode itself.

[0171] The node 1-4 a having received this message writes the upstreamport number 10 for the alternate path 21-1 in the routing table 18,selects the downstream port and write it in the routing table 18, andthen rewrites the upstream port number of the downstream node in thesetting request message and send it to a node 1-6 a.

[0172] The node 1-6 a similarly sets the routing table, and thentransfers the setting request message to the node 1-5 a.

[0173] The node 1-5 a having received the setting request message of thealternate path 21-1 first writes the received upstream port number inthe routing table 18. The node 1-5 a selects the downstream node numberand downstream port number as in the primary path 20-1, and writes themin the routing table 18. Then, the node 1-5 a rewrites the upstream portnumber of the downstream node of the setting request message and sendsit to the node 1-2 b that is the downstream node (S116).

[0174] In this way, setting of the routing table 18 for the primary path20-1 and alternate path 21-1 in the subnetwork 3-a is completed (S117).

[0175] On the other hand, the topology table 12 in the subnetwork 3-bstores topology information in the subnetwork 3-b, the contents of FIG.4, and information that the node 1-2 b is connected to the node 1-5 a ofthe subnetwork 3-a. The SRLG table 16 in the management center 2 of thesubnetwork 3-b holds SRLG information in the subnetwork 3-b, that is,the same contents as FIG. 5. In the path table 17, routes and SRLGs ofthe path in the subnetwork 3-b are recorded.

[0176] In the subnetwork 3-b, the node 1-2 b first receives the settingrequest message of the primary path 20-1 from the node 1-5 a (S12) (S121in FIG. 37). This message indicates that the destination node of thispath is the node 1-5 b in the same subnetwork, but does not indicate theroute thereto. Thus, the node 1-2 b refers to its own topology table 12to calculate the shortest route from the node 1-2 b to node 1-5 b thatsimply passes link groups having one or more available links, using theCSPF algorithm (S81 in FIG. 32). This provides the routes (2, 6), (5, 6)to be the routes of the primary path 20-1. Then, the node 1-2 b sendsthe routes obtained to a management center 2-b (S82). A centralizedcontrol unit 15 of the management center 2-b records the routes in thepath table 17. Next, the centralized control unit 15 refers to the SRLGtable 16 to prepare a list of a link group not belonging to SRLG thatthe routes of the primary path 20-1 sent from the node 1-2 b pass, andreturns the list to the node 1-2 b (S83). Here, a list of link groups(1, 2), (1, 3), (1, 4), (3, 5), (4, 5), (4, 6) is sent. Then, the node1-2 b calculates the shortest route from the node 1-2 b to node 1-5 bsimply using link groups having one or more available links among linkgroups included in the list (S84). This provides the routes (1, 2), (1,3), (3, 5) to be the routes of the alternate path 21-1.

[0177] The node 1-2 b also sends the routes of the alternate path 21-1to the management center 2-b (S85). The management center 2-b searchesfor an alternate path having a route overlapping the route of thealternate path 21-1 in the path table 17, but there is no such alternatepath at this time (N in S87). Thus, the management center 2-b sends amessage indicating “share no resource” to the node 1-2 b (S88), andrecords the routes of the alternate path 21-1 in the path table 17.

[0178] Subsequently, the node 1-2 b sets the routing table 18 for theprimary path 20-1 (S123 in FIG. 37), and then writes the route from thenode 1-2 b to the node 1-5 b in the setting request message, and send itto the node 1-6 b that is the downstream node. The node 1-6 b also setsthe routing table 18, and then rewrites the setting request message andsent it to the node 1-5 b. In these nodes, setting of the routing table18 and signaling of the setting request message are performed incompletely the same manner as performed for the primary path 20-1 in thesubnetwork 3-a.

[0179] Finally, the node 1-5 b having received the setting requestmessage sets the upstream port of the routing table 18 as specified bythe node 1-6 b (S124). The node 1-5 b is the destination node of theprimary path 20-1, so that the downstream node is the client, and theport having the smallest port number is selected from the portsconnected to the client as the downstream port. The node 1-5 b alsowrites this information in the routing table 18. The node 1-5 b changesa switch 10 in accordance with the contents of the routing table 18, andthen generates a setting response message that is a kind of signalingmessages and sends it to the node 1-6 b. The setting response messageincludes information such as identification data of the message,identification data indicating that this path is the primary path, pathnumber, source node number, and destination node number. The settingresponse message is transferred in a direction opposite the path on theroute of the primary path 20-1 to the node 1-1 a that is the source node(S125). Each node on the route receives the setting response message tochange its own switch 10 in accordance with the contents of the routingtable 18. In this way, setting of the primary path 20-1 is completed(S126).

[0180] Then, the node 1-2 b sets the routing table 18 for the alternatepath 21-1 (S127), writes the route information of the alternate path21-1 in the subnetwork 3-b in the setting request message, and sends itto the node 1-1 b. The setting request message is transferred from thenode 1-1 b to the node 1-3 b and node 1-5 b, and the routing table 18 isset in each node on the way (S128). The source is not shared withanother alternate path, and setting of the routing table 18 andsignaling of the setting request message are performed in completely thesame manner as performed for the alternate path 21-1 in the subnetwork3-a.

[0181] In the node 1-5 b, the same port as allocated to the primary path20-1 is allocated as the downstream port for the alternate path 21-1.When setting of the routing table 18 is completed, the node 1-5 bgenerates the setting response message. This message is transferred in adirection opposite the path on the route of the alternate path 21-1 tothe node 1-1 a that is the source node (S129). Each node on the routedoes not change the switch 10 when it receives the setting responsemessage to the alternate path. In this way, setting of the alternatepath 21-1 is completed (S130).

[0182] Next, setting a primary path 20-2 and alternate path 21-2 whosesource is the node 1-1 a in the subnetwork 3-a and whose destination isa node 1-6 c in a subnetwork 3-c will be considered.

[0183] The node control unit 11 in the node 1-1 a first refers to theexternal routing table 60 to find that the route may pass the node 1-6 awhen the path whose destination is the node in the subnetwork 3-c (S111in FIG. 36) is set. The node control unit 11 of the node 1-1 acalculates the shortest route from the node 1-1 a to node 1-6 a thatsimply passes link groups having one or more available links (S81 inFIG. 32). This provides the routes (1, 2), (2, 6) of the primary path20-2.

[0184] Then, the node 1-1 a sends the routes obtained of primary path20-1 to the management center 2-a (S82). The centralized control unit 15of the management center 2-a records the routes in the path table 17.Next, the centralized control unit 15 refers to the SRLG table 16 toprepare a list of a link group not belonging to SRLG that the routes ofthe primary path 20-2 sent from the node 1-1 a pass, and returns thelist to the node 1-1 a (S83). Here, a list of link groups (1, 3), (1,4), (3, 5), (4, 5), (4, 6), (5, 6) is sent. The node 1-1 a calculatesthe shortest route from the node 1-1 a to node 1-6 a simply using linkgroups having one or more available links among link groups included inthe list (S84). This provides the routes (1, 4), (4, 6) to be paths ofthe alternate path 21-2.

[0185] The node 1-1 a also sends the routes of the alternate path 21-2to the management center 2-a (S85). The centralized control unit 15 ofthe management center 2 searches for an alternate path having a routeoverlapping the route of the alternate path 21-2 in the path table 17(S86). The alternate path 21-1 overlaps the alternate path 21-2 in thelink groups (1, 4), (4, 6) (Y in S87) here. Thus, the centralizedcontrol unit 15 checks the SRLGs that the routes of the primary path20-1 and primary path 20-2 corresponding to the alternate paths pass(S17 in FIG. 26). The SRLGs of both primary paths do not overlap (N inS18), so that the centralized control unit 15 sends a message indicating“share the link with the alternate path 21-1 in the link groups (1, 4),(4, 6)” to the node 1-1 a, and records the routes of the alternate path21-2 in the path table 17 (Sl9).

[0186] Subsequently, the node 1-1 a sets the routing table 18 for theprimary path 20-2 (S20, S21), and then generates the setting requestmessage. The setting request message is transferred to the node 1-6 avia the node 1-4 a (S22). In accordance therewith, the routing tablesfor the 20-2 are also set in the node 1-4 a and node 1-6 a. The node 1-6a is the boundary node, so that the node 1-1 c is set as the downstreamnode, and the port connected to the primary link group 40-ac is set asthe downstream port in the routing table 18. Then, the node 1-6 atransfers the setting request message to the node 1-1 c. A series ofsteps as described above is performed in completely the same manner asperformed in the subnetwork 3-a for the primary path 20-1.

[0187] Next, the node 1-1 a sets the routing table 18 for the 21-2(S23). The node 1-1 a selects the same port as allocated to the 21-1, asthe downstream port. Then, the node 1-1 a generates the setting responsemessage to the alternate path 21-2 (S24). The setting request messageincludes information such as identification data of the message,identification data indicating that this path is the alternate path,path number, source node number, destination node number, routeinformation to the node 1-6 a, and upstream port number of thedownstream port, and also information that this path shares the linkwith the alternate path 21-1 in the link groups (1, 4), (4, 6).

[0188] The node 1-4 a having received the setting request messageselects the same port as allocated to the alternate path 21-1, as thedownstream port for the alternate path 21-2. Further, the node 1-4 awrites the port number of the node 1-6 a connected to the downstreamport in the setting request message, and transfers it to the node 1-6 a.

[0189] The node 1-6 a writes the port indicated from the node 1-4 a bythe setting request message in the routing table 18 as the upstreamport. The node 1-6 a is the boundary node, so that the same downstreamnode and downstream port as set for the primary path 20-2 are set. Then,the node 1-6 a transfers the setting request message to the node 1-1 c(S113 to S117 in FIG. 36).

[0190] In this way, setting of the routing table 18 for the primary path20-2 and alternate path 21-2 in the subnetwork 3-a is completed.

[0191] Then, setting for the primary path 20-2 and alternate path 21-2in the subnetwork 3-c is performed.

[0192] First, the node 1-1 c having received the setting request messageto the primary path 20-2 from the node 1-6 a (S121 in FIG. 37)calculates the route of the primary path 20-2 from the node 1-1 c tonode 1-6 c (S81 in FIG. 32) and send the route to a management center2-c (S82). A centralized control unit 15 of the management center 2-crecords the route in the path table 17 to prepare a list of a link groupnot belonging to SRLG that the route of the primary path 20-2 sent fromthe node 1-1 c pass, and returns the list to the node 1-1 c (S83). Then,the node 1-1 c calculates the route of the alternate path 21-2 from thenode 1-1 c to node 1-6 c simply using the link groups included in thislist (S84). Here, the contents of the SRLG table or path table in themanagement center 2-c are the same as in the management center 2-a, sothat a calculation method of the routes of the primary path 20-2 andalternate path 21-2 in the subnetwork 3-c and the obtained results areidentical to those of the routes of the primary path 20-2 and alternatepath 21-2 in the subnetwork 3-a.

[0193] The node 1-1 c also sends the routes of the alternate path 21-2to the management center 2-c (S85). The management center 2-c searchesfor an alternate path having a route overlapping the route of thealternate path 21-2 in the path table 17, but there is no such alternatepath at this time (N in S87). Thus, the management center 2-c sends amessage indicating “share no resource” to the node 1-1 c, and recordsthe route of the alternate path 21-2 in the path table 17 (S88).

[0194] Subsequently, the node 1-1 c sets the routing table 18 for theprimary path 20-2 (S123 in FIG. 37), and then writes the route of theprimary path 20-2 in the subnetwork 3-c in the setting request message,and sends it to the node 1-2 c. The setting request message istransferred from the node 1-2 c to the node 1-6 c (S124). In accordancetherewith, the routing tables 18 for the 20-2 are also set in the node1-2 c and node 1-6 c. The node 1-6 c is the destination node, so thatthe client is selected as the downstream node, and the port having thesmallest port number is selected from unused ports connected to theclient as the downstream port, and these are set in the routing table18. Then, the node 1-6 c generates the setting response message and sendit to the node 1-2 c. The setting response message is transferred in adirection opposite the path on the primary path 20-2 to the node 1-1 athat is the source node (S125). In accordance therewith, each node onthe route changes the switch 10 (S126).

[0195] Then, the node 1-1 c sets the routing table 18 for the alternatepath 21-2 (S127), and then writes the route of the alternate path 21-2in the subnetwork 3-c in the setting request message, and sends it tothe node 1-4 c. The setting request message is transferred from the node1-4 c to the node 1-6 c (S128). In accordance therewith, the routingtables 18 for the alternate path 21-2 are also set in the node 1-4 c andnode 1-6 c, but the link is not shared with another alternate path. Thenode 1-6 c is the destination node, so that the client is selected asthe downstream node, and the port having the smallest port number isselected from unused ports connected to the client as the downstreamport, and these are set in the routing table 18. Then, the node 1-6 cgenerates the setting response message and send it to the node 1-4 c.The setting response message is transferred in a direction opposite thepath on the alternate path 21-2 to the node 1-1 a that is the sourcenode (S129). Each node on the path do not change the switch 10 when itreceives the setting response message to the alternate path. In thisway, setting of the primary path 20-2 and alternate path 21-2 iscompleted (S130).

[0196] According to this embodiment, two pairs of path, that is, theprimary path 20-1 and alternate path 21-1, and the primary path 20-2 andalternate path 21-2 can be set so as not to share the SRLG in eachsubnetwork 3. Thus, even if failure occurs in the link or node on theprimary path 20-1 or primary path 20-2 in each subnetwork 3, the failurecan be recovered by switching to the alternate path. At the boundary ofeach subnetwork 3, failure recovery is also performed by APS. Further,in the subnetwork 3-a, the alternate path 21-1 and alternate path 21-2share the link on the link groups (1, 4), (4, 6), thereby allowingsavings in alternate sources.

[0197] Next, an eighth embodiment will be described. In the eighthembodiment, a network consists of a plurality of subnetworks as in theseventh embodiment, and there is no management center 2. A configurationof a network is shown in FIG. 22. The configuration of the network iscompletely the same as that in the seventh embodiment except for absenceof the management center 2. A configuration of a node 1 is shown in FIG.23. In this embodiment, the node 1 has an external routing table 60 asin the seventh embodiment. Other than that, the configuration of thenode 1 is completely the same as that in the third embodiment, andincludes an SRLG table 16 and a path table 17.

[0198] Thus, receiving/transmission of messages between the subnetworksare the same as in the flowcharts of the seventh embodiment, andoperation of the node 1 is the same as in the flowcharts of the thirdembodiment. Therefore, description of the operation using the flowchartswill be omitted in the eighth embodiment.

[0199] First, setting a primary path 20-1 and alternate path 21-1 whosesource is a node 1-1 a in a subnetwork 3-a and whose destination is anode 1-5 b in a subnetwork 3-b will be considered.

[0200] A node control unit 11 of the node 1-1 a first refers to theexternal routing table 60 to find that a route may pass a node 1-5 awhen the path whose destination is the node in the subnetwork 3-b isset. The node control unit 11 of the node 1-1 a refers to a topologytable 12 and the SRLG table 16 to calculate the routes from the node 1-1a to node 1-5 a of the primary path 20-1 and alternate path 21-1 in thesame manner as in the second embodiment. This provides the routes (1,3), (3, 5) of the primary path 20-1, and the routes (1, 4), (4, 6), (5,6) of the alternate path 21-1, and these routes share no SRLG. The node1-1 a records the routes and passing SRLGs in the path table 17.

[0201] Then, the node 1-1 a sets a routing table 18 for the primary path20-1. A setting manner here is the same as in the seventh embodiment.

[0202] Next, the node 1-1 a sends a setting request message of theprimary path 20-1 that is a kind of signaling messages to the node 1-3 avia a control channel 31. The setting request message includesinformation such as identification data of the message, identificationdata indicating that this path is the primary path, path number, sourcenode number, destination node number, route information and SRLGinformation from the node 1-1 a to node 1-5 a, and upstream port numberof the downstream port. For the SRLG information from the node 1-1 a tothe node 1-5 a, all the numbers of the SRLGs are added to which linkgroups that this path passes in the subnetwork 3-a belong. Other thanthat, the setting request message is the same as that in the seventhembodiment.

[0203] The node 1-3 a having received the setting request message setsthe routing table 18 as in the seventh embodiment, rewrites the upstreamport number of the downstream node in the setting request message, andsends the setting request message to the node 1-5 a. The node 1-3 awrites the route information and SRLG information of the primary path20-1 indicated by the setting request message in its own path table 17.

[0204] The node 1-5 a having received the setting request message setsthe routing table 18 as in the seventh embodiment, rewrites the upstreamport number of the downstream node in the setting request message, andsends the setting request message to the node 1-2 b. The node 1-5 awrites the route information and SRLG information of the primary path20-1 indicated by the setting request message in its own path table 17.

[0205] After setting the primary path 20-1, the node 1-1 a sets therouting table 18 for the alternate path 21-1. At this time, the node 1-1a refers to the path table 17 and searches for another alternate pathpassing the link group (1, 4) like the alternate path 21-1, but there isno such alternate path. Thus, the node 1-1 a selects the port 10 havingthe smallest port number from unused ports connected to the link group(1, 4) as the downstream port, and writes it in the routing table 18.

[0206] Subsequently, the node 1-1 a sends the setting request message ofthe alternate path 21-1 to the node 1-4 a via the control channel 31.The setting request message includes information such as identificationdata of the message, identification data indicating that this path isthe alternate path, path number, source node number, destination nodenumber, route information and SRLG information from the node 1-1 a tonode 1-5 a, and upstream port number of the downstream port.

[0207] The node 1-4 a having received the message refers to the pathtable 17 to search for another alternate path passing the link group (4,6) like the alternate path 21-1, but there is no such alternate path.Thus, the node 1-4 a selects the port having the smallest port numberfrom unused ports connected to the link groups (4, 6) as the downstreamport, and writes it in its own routing table 18. The node 1-4 a rewritesthe upstream port of the downstream node of the setting request message,and transfers it to the node 1-6 a. Further, the node 1-4 a writes theroute information and SRLG information of the alternate path 21-1indicated by the setting request message in its own path table 17.

[0208] The node 1-6 a having received the setting request message refersto the path table 17 to search for another alternate path passing thelink group (5, 6) like the alternate path 21-1, but there is no suchalternate path. Thus, the node 1-6 a selects the port having thesmallest port number from unused ports connected to the link group (5,6) as the downstream port, and writes it in its own routing table 18.The node 1-6 a rewrites the upstream port of the downstream node of thesetting request message, and transfers it to the node 1-5 a. Further,the node 1-6 a writes the route information and SRLG information of thealternate path 21-1 indicated by the setting request message in its ownpath table 17.

[0209] The node 1-5 a having received the setting request message knowsthat the node 1-5 a itself is the boundary node, and thus allocates thesame port as allocated to the downstream port of the alternate path 20-1to the downstream port of the alternate path 21-1, and sets the routingtable 18. The node 1-5 a rewrites the upstream port number of thedownstream node of the setting request message, and transfers it to thenode 1-2 b. Further, the node 1-5 a writes the route information andSRLG information of the alternate path 21-1 indicated by the settingrequest message in its own path table 17.

[0210] In this way, setting of the routing table 18 for the primary path20-1 and alternate path 21-1 in the subnetwork 3-a is completed.

[0211] On the other hand, the topology table 12 in the subnetwork 3-bstores topology information in the subnetwork 3-b, the contents of FIG.4, and information that the node 1-2 b is connected to the node 5 a ofthe subnetwork 3-a. The SRLG table 16 in the subnetwork 3-b holds SRLGinformation in the subnetwork 3-b, that is, the same contents as FIG. 5.In the path table 17, routes and SRLGs of the path in the subnetwork 3-bare recorded.

[0212] In the subnetwork 3-b, the node 1-2 b first receives the settingrequest message of the primary path 20-1 from the node 1-5 a. Thismessage indicates that the destination node of this path is the node 1-5b in the same subnetwork, but does not indicate the route thereto. Thus,the node 1-2 b refers to the topology table 12 and SRLG table 16 tocalculate the routes from the node 1-2 b to node 1-5 b of the primarypath 20-1 and alternate path 21-1. This provides the routes (2, 6), (5,6) of the primary path 20-1, and the routes (1, 2), (1, 3), (3, 5) ofthe alternate path 21-1, and these routes share no SRLG. The node 1-2 brecords the route and passing SRLG in the path table 17.

[0213] Then, the node 1-2 b sets the routing table 18 for the primarypath 20-1. A setting manner here is the same as in the seventhembodiment.

[0214] Next, the node 1-2 b rewrites the received route information andSRLG information of the setting request message of the primary path 20-1into the route information and SRLG information in the subnetwork 3-b,and also rewrites the upstream port number of the downstream node totransmit it to the node 1-6 b.

[0215] The node 1-6 b having received the setting request message setsthe routing table 18 as in the seventh embodiment, rewrites the upstreamport number of the downstream node in the setting request message, andsend the setting request message to the node 1-5 b. The node 1-6 bwrites the route information and SRLG information of the primary path20-1 indicated by the setting request message in its own path table 17.

[0216] The node 1-5 b having received the setting request message setsthe routing table 18 as in the seventh embodiment, changes the switch 10in accordance with the contents of the routing table 18, generates thesetting response message, and send it to the node 1-6 b. The settingresponse message is transferred in a direction opposite the path on theroute of the primary path 20-1 to the node 1-1 a that is the sourcenode. Each node on the routes receives the setting response message tochange its own switch 10 in accordance with the contents of the routingtable 18. In this way, setting of the primary path 20-1 is completed.

[0217] Then, the node 1-2 b sets the routing table 18 for the alternatepath 21-1. First, the node 1-2 b refers to the path table 17 andsearches for another alternate path passing the link group (1, 2) likethe alternate path 21-1, but there is no such alternate path. Thus, thenode 1-2 b selects the port having the smallest port number from unusedports connected to the link groups (1, 2) as the downstream port, andwrites it in its own routing table 18. The node 1-2 b rewrites theupstream port of the downstream node of the setting request message, andtransfers it to the node 1-1 b. Further, the node 1-2 b writes the routeinformation of the alternate path 21-1 and SRLG information indicated bythe setting request message in its own path table 17.

[0218] The node 1-1 b having received the setting, request messagerefers to the path table 17 to search for another alternate path passingthe link group (1, 3) like the alternate path 21-1, but there is no suchalternate path. Thus, the node 1-1 b selects the port having thesmallest port number from unused ports connected to the link groups (1,3) as the downstream port, and writes it in its own routing table 18.The node 1-1 b rewrites the upstream port of the downstream node of thesetting request message, and transfers it to the node 1-3 b. Further,the node 1-1 b writes the route information and SRLG information of thealternate path 21-1 indicated by the setting request message in its ownpath table 17.

[0219] The node 1-3 b having received the setting request message refersto the path table 17 to search for another alternate path passing a linkgroup (3, 5) like the alternate path 21-1, but there is no suchalternate path. Thus, the node 1-3 b selects the port having thesmallest port number from unused ports connected to the link groups (3,5) as the downstream port, and writes it in its own routing table 18.The node 1-3 b rewrites the upstream port of the downstream node of thesetting request message, and transfers it to the node 1-5 b. Further,the node 1-3 b writes the route information and SRLG information of thealternate path 21-1 indicated by the setting request message in its ownpath table 17.

[0220] The node 1-5 b having received the setting request message is thedestination node of the alternate path 21-1, and thus allocates the sameport as allocated to the downstream port of the primary path 20-1 to thedownstream port of the alternate path 21-1, and sets the routing table18. Then, the node 1-5 b generates the setting response message, andsend it to the node 1-3 b. The setting response message is transferredin a direction opposite the path on the route of the alternate path 21-1to the node 1-1 a that is the source node. This setting response messageis for the alternate path, and thus each node on the route does notchange the switch 10. In this way, setting of the alternate path 21-1 iscompleted.

[0221] Next, setting a primary path 20-2 and alternate path 21-2 whosesource is the node 1-1 a in subnetwork 3-a and whose destination is anode 1-6 c in a subnetwork 3-c will be considered.

[0222] The node control unit 11 of the node 1-1 a first refers to theexternal routing table 60 to find that the route may pass the node 1-6 awhen the path whose destination is the node in the subnetwork 3-c isset. The node control unit 11 of the node 1-1 a refers to the topologytable 12 and SRLG table 16 to calculate the routes from the node 1-1 ato node 1-6 a of the primary path 20-2 and alternate path 21-2 in thesame manner as in the second embodiment. This provides the routes (1,2), (2, 6) of the primary path 20-2, and the routes (1, 4), (4, 6) ofthe alternate path 21-2, and these routes share no SRLG. The node 1-1 arecords the routes and passing SRLGs in the path table 17.

[0223] Then, the node 1-1 a sets the routing table 18 for the primarypath 20-2, and generates the setting request message of the primary path20-2. The setting request message is transferred from the node 1-1 a tonode 1-2 a, and node 1-6 a, and in accordance therewith, the routingtable 18 is set, and the route information and SRLG information arewritten in the path table 17 also in the node 1-2 a and node 1-6 a. Thenode 1-6 a that is the boundary node selects the port connected to thelink group 40-ac as the downstream port, and transfers the settingrequest message to the node 1-1 c. The above described steps areperformed in completely the same manner as for the primary path 20-1 inthe subnetwork 3-a.

[0224] Then, the node 1-1 a sets the routing table 18 for the alternatepath 21-2. First, the node 1-1 a searches the path table 17 and checkswhether there is another alternate path passing the link group (1, 4).The alternate path 21-1 applies thereto here, so that the node 1-1 acompares the SRLGs in the subnetwork 3-a that the primary path 20-1 andprimary path 20-2 pass. The comparison reveals that both SRLGs do notoverlap, and thus the node 1-1 a selects the same port as allocated tothe alternate path 21-1, as the downstream port of the alternate path21-2. That is, the two alternate paths share the link in the link group(1, 4). Then, the node 1-1 a generates the setting request message ofthe alternate path 21-2 and sends it to the node 1-4 a. This settingrequest message includes the paths information and SRLG information ofthe alternate path 21-2 in the subnetwork 3-a.

[0225] The node 1-4 a having received the setting request messagesearches its own path table 17 and checks whether there is anotheralternate path passing the link group (4, 6). The alternate path 21-1also applies thereto here, so that the node 1-4 a compares the SRLGs inthe subnetwork 3-a that the primary path 20-1 and primary path 20-2pass. Both SRLGs do not overlap, so that the node 1-4 a selects the sameport as allocated to the alternate path 21-1, as the downstream port ofthe alternate path 21-2. That is, the two alternate paths share the linkalso in the link group (4, 6). The node 1-4 a writes the pathsinformation and SRLG information of the alternate path 21-2 indicated bythe setting request message in its own path table 17.

[0226] The node 1-6 a having received the setting request message is theboundary node, and thus selects the same port as allocated to theprimary path 20-2, as the downstream port of the alternate path 21-2,and sets it in the routing table 18. The node 1-6 a writes the pathsinformation and SRLG information of the alternate path 21-2 indicated bythe setting request message in its own path table 17. Further, the node1-6 a rewrites the upstream port of the downstream node in the settingrequest message, and transfers it to the node 1-1 c.

[0227] In this way, setting of the routing table 18 for the primary path20-2 and alternate path 21-2 in the subnetwork 3-a is completed.

[0228] On the other hand, in the subnetwork 3-c, the node 1-1 c receivesthe setting request message of the primary path 20-2 from the node 1-6a. This message indicates that the destination node of this path is thenode 1-6 c in the same subnetwork, but does not indicate the routethereto. Thus, the node 1-1 c refers to the topology table 12 and SRLGtable 16 to calculate the routes from the node 1-1 c to node 1-6 c ofthe primary path 20-2 and alternate path 21-2. This provides the routes(1, 2), (2, 6) of the primary path 20-2, and the routes (1, 4), (4, 6)of the alternate path 21-2, and these routes share no SRLG. The node 1-1c records the routes and passing SRLGs in the path table 17.

[0229] Then, the node 1-1 c sets the routing table 18 for the primarypath 20-2, and generates the setting request message. The settingrequest message is transferred from the node 1-1 c to the node 1-2 c andnode 1-6 c, and in accordance therewith, the routing table 18 is set,and the route information and SRLG information are written in the pathtable 17 also in the node 1-2 c and node 1-6 c. The node 1-6 c is thedestination node, and when setting the routing table 18, the node 1-6 cchanges the switch 10 in accordance therewith to generate the settingresponse message. The setting response message is transferred in adirection opposite the path on the route of the primary path 20-2 to thenode 1-1 a that is the source node. Each node on the route receives thesetting response message to change its own switch 10 in accordance withthe contents of the routing table 18. The above described steps areperformed in completely the same manner as for the primary path 20-1 inthe subnetwork 3-b. Therefore, setting of the primary path 20-2 iscompleted.

[0230] Then, the node 1-1 c sets the routing table 18 for the alternatepath 21-2. First, the node 1-1 c refers to the path table 17 andsearches for another alternate path passing a link group (1, 4) like thealternate path 21-2, but there is no such alternate path. Thus, the node1-1 c selects the port having the smallest port number from unused portsconnected to the link group (1, 4) as the downstream port, and writes itin its own routing table 18. The node 1-1 c rewrites the upstream portof the downstream node of the setting request message, and transfers itto the node 1-4 c. Further, the node 1-1 c writes the route informationand SRLG information of the alternate path 21-2 indicated by the settingrequest message in its own path table 17.

[0231] The node 1-4 c having received the setting request message refersto the path table 17 to search for another alternate path passing thelink group (4, 6) like the alternate path 21-2, but there is no suchalternate path. Thus, the node 1-4 c selects the port having thesmallest port number from unused ports connected to the link group (4,6) as the downstream port, and writes it in its own routing table 18.The node 1-4 c rewrites the upstream port of the downstream node of thesetting request message, and transfers it to the node 1-6 c. Further,the node 1-4 c writes the route information and SRLG information of thealternate path 21-2 indicated by the setting request message in its ownpath table 17.

[0232] The node 1-6 c having received the setting request message is thedestination node of the alternate path 21-2, and thus allocates the sameport as allocated to the downstream port of the primary path 20-2 to thedownstream port of the alternate path 21-2, and sets the routing table18. Then, the node 1-6 c generates the setting response message, andsend it to the node 1-3 c. The setting response message is transferredin a direction opposite the path on the route of the alternate path 21-2to the node 1-1 a that is the source node. This setting response messageis for the alternate path, and thus each node on the route does notchange the switch 10. In this way, setting of the alternate path 21-2 iscompleted.

[0233] According to this embodiment, two pairs of path, that is theprimary path 20-1 and alternate path 21-1, and the primary path 20-2 andalternate path 21-2 can be set so as not to share the SRLG in eachsubnetwork 3. Thus, even if failure occurs in the link or node on theprimary path 20-1 or primary path 20-2 in each subnetwork 3, the failurecan be recovered by switching to the alternate path. At the boundary ofeach subnetwork 3, failure recovery is also performed by APS. Further,in the subnetwork 3-a, the alternate path 21-1 and alternate path 21-2shares the link on the link groups (1, 4), (4, 6), thereby allowingsavings in alternate sources.

[0234] In this embodiment, the above described advantages are obtainedby decentralized control without centralized controlling means.

[0235] The present invention can be applied to the embodiments describedbelow.

[0236] 1. In the above described embodiments, the two-way link is used,but a one-way way link may be used.

[0237] 2. It is described that there are a plurality of links as a linkgroup, but not limited to the plurality of links, there may be a singlelink.

[0238] 3. In the above described embodiments, a path setting method inconsideration of SRLG as a risk sharing group is described, but the risksharing group is not limited to the SRLG. For example, a path settingmethod in consideration of a risk sharing group such as “a group ofnodes sharing a resource” may be possible.

[0239] 4. In the above described embodiment, a method for setting theprimary path and alternate path between two nodes is described, butdifferentiation between the primary path and alternate path is notalways necessary. For example, both two paths may be primary paths andused for load decentralization (applicable to the fifth embodiment).

[0240] 5. In the above described embodiments, it is described that thesource nodes of the plurality of primary and alternate paths that sharethe alternate resource (link) are the same, but not limited to this, thepaths may be set in similar steps when, for example, a source node of afirst pair of primary and alternate paths are a node 1-1, and a sourcenode of a second pair of primary and alternate paths are a node 1-2.

[0241] 6. In the above described embodiments, there are two pairs ofprimary and alternate paths that share the alternate resource (link),but not limited to this, there maybe three or more pairs. Setting stepsin that case are the same as those of the first pairs of paths and thesecond pairs of paths (though the results are different due to thedifferent conditions). For example, repeating the steps for five pairs,five pairs of primary and alternate paths may share the alternateresource.

[0242] As described above, in the seventh and eight embodiments,signaling of the setting request message is performed from the sourcenode via the midstream nodes to the destination node, and after therouting table is set in the destination node, the setting responsemessage is transferred from the destination node to the source node onthe same route in the opposite direction. Each node on the routereceives the setting response message, changes its own switch inaccordance with the contents of the routing table, and in this way,setting of the primary and alternate paths is completed (for the seventhembodiment, see page 46, lines 1 to 21, and page 47, lines 5 to 14, andfor the eighth embodiment, see page 58, lines 17 to 27, page 60, lines10 to 22, page 63, line 18 to page 64, line 7, and page 65, lines 5 to17).

[0243] In the first to sixth embodiments, described steps are signalingof the setting request message from the source node via the midstreamnodes to the destination node, and setting of the routing table in thedestination node, for convenience of explanation. However, actually,after the routing table is set in the destination node, the settingresponse message is transferred from the destination node to the sourcenode on the same route in the opposite direction as in the seventh andeighth embodiments.

[0244] Next, a ninth embodiment will be described. The ninth embodimentrelates to a recording medium having a path setting program recordedthereon. The path setting program is a program to perform steps shown inthe flowcharts of FIGS. 24 to 38 in a computer. Each step of theflowchart with indication of (N) at its front is a node control program,and each step with indication of (K) is a management center controlprogram. These programs are recorded in the recording medium.

[0245] Then, a configuration of a unit controlled by the path settingprogram will be described. FIG. 39 shows a configuration of a nodecontrolled by the path setting program, and FIG. 40 shows aconfiguration of a management center controlled by the path settingprogram.

[0246] First, a configuration of the node will be described. Withreference to FIG. 39, a node 1 comprises a CPU (central processing unit)101 in addition to a node control unit 11. The CPU 101 reads a programfrom a recording medium having a path setting program recorded thereon102N, and controls the node control unit 11 in the node 1. The contentsof control is described above, and the description thereof will beomitted.

[0247] Next, a configuration of the management center will be described.With reference to FIG. 40, the management center 2 comprises a CPU 103in addition to a centralized control unit 15. The CPU 103 reads aprogram from a recording medium having a path setting program recordedthereon 102K, and controls the centralized control unit 15 in themanagement center 2. The contents of control are also described above,and the description thereof will be omitted.

[0248] The first aspect of the invention provides a communicationnetwork including a plurality of nodes constituting a network and amanagement center connected to each of the nodes, wherein each of thenodes has topology information of the network, and the management centerhas information on a risk sharing resource group, thereby allowing loadof route calculation to be decentralized to the nodes and the managementcenter. This prevents the load of the route calculation from beingcentralized in part of units and prevents increase in traffic betweenthe nodes.

[0249] The second to sixth aspects of the invention achieves the sameadvantages as the first aspect. Further, advantages obtained from eachembodiment are summarized as described below.

[0250] In the first and fourth embodiments, each of the nodes hastopology information of the network, and the management center hasinformation on the risk sharing resource group and currently set pathinformation, so that each node no longer needs to hold the informationon the risk sharing source and path information. Therefore, load on eachnode can be reduced.

[0251] In the first embodiment, information on whether SRLGs overlap isobtained from the management center after routes of the primary andalternate paths are calculated, so that several times of calculation issometimes required before the primary and alternate paths having nooverlapping SRLG can be obtained. However, in the fourth embodiment, alist of SRLGs not included in the route of the primary path is receivedfrom the management center before the route of the alternate path iscalculated, thereby always allowing single calculation of the alternatepath.

[0252] In the second embodiment, each node has topology information ofthe network and information on the risk sharing resource group, and themanagement center has currently set path information, so that each nodeno longer needs to hold the path information. Therefore, load on eachnode can be reduced.

[0253] In the third embodiment, each node has topology information ofthe network, information on the risk sharing resource group, andinformation on a currently set path passing the node itself, therebyeliminating the need for the management center, and preventing thenumber of nodes of the network from being limited by capacity of themanagement center. Failure of the management center do not cause theentire network to be down. Further, each node holds only information onthe path passing the node itself as path information, so that a largememory for each node is not required. Also, each node searches for onlyexisting alternate paths having the overlapping route in the link groupbetween the node itself and the downstream node, thereby allowing loadof searching for an existing path on each node to be reduced.

[0254] In the fifth embodiment, each node has topology information ofthe network, and the management center has information on the risksharing resource group, so that each node no longer needs to hold theinformation on the risk sharing resource group. Therefore, load on eachnode can be reduced.

[0255] In the sixth embodiment, each node has topology information ofthe network, and the management center has currently set pathinformation, so that each node no longer needs to hold the pathinformation. Therefore, load on each node can be reduced.

[0256] In the seventh and eighth embodiments, the network shown in thefirst to sixth embodiments consists of a plurality of subnetworks,thereby allowing the primary path and alternate path closed for eachsubnetwork to be set. Therefore, failure recovery is performed for eachsubnetwork, and failure recovery time is reduced.

What is claimed is:
 1. A communication network comprising: a pluralityof nodes constituting a network; and a management center connected toeach of said nodes, wherein each of said nodes has topology informationof said network, and said management center has information on a risksharing resource group.
 2. A communication network comprising: aplurality of nodes constituting a network; and a management centerconnected to each of said nodes, wherein each of said nodes has topologyinformation of said network, and said management center has currentlyset path information.
 3. A communication network comprising: a pluralityof nodes constituting a network; and a management center connected toeach of said nodes, wherein each of said nodes has topology informationof said network, and said management center has information on a risksharing resource group and currently set path information.
 4. Acommunication network comprising: a plurality of nodes constituting anetwork; and a management center connected to each of said nodes,wherein each of said nodes has topology information of said network andinformation on a risk sharing resource group, and said management centerhas currently set path information.
 5. A communication networkcomprising: a plurality of nodes constituting a network, wherein each ofsaid nodes has topology information of said network, information on arisk sharing resource group, and information on a currently set pathpassing the node itself.
 6. A communication network in which acommunication network as set forth in claim 3 consists of a plurality ofsubnetworks, wherein each of nodes comprises an external routing tableshowing a boundary node that a route passes when a path to a destinationnode in another subnetwork is set.
 7. A communication network in which acommunication network as set forth in claim 5 consists of a plurality ofsubnetworks, wherein each of nodes comprises an external routing tableshowing a boundary node that a route passes when a path to a destinationnode in another subnetwork is set.
 8. The communication networkaccording to claim 1, wherein when a first path and a second path havingdifferent routes are calculated, said information on the risk sharingresource group is referred, and paths having no overlapping risk sharingresource group are determined.
 9. The communication network according toclaim 2, wherein when a plurality of pairs of first paths and secondpaths having different routes are calculated, said path information isreferred, and a plurality of said second paths having an overlappingroute is searched.
 10. The communication network according to claim 9,wherein when said second paths having the overlapping route exist, saidinformation on the risk sharing resource group is referred, and it ischecked whether the first paths pairing up with said respective secondpaths have an overlapping risk sharing resource group.
 11. Thecommunication network according to claim 10, wherein each of said nodesare linked by a link group consisting of one or more links, and when thefirst paths pairing up with said respective second path have nooverlapping risk sharing resource group, said plurality of second pathshaving the overlapping route share a link on said link group.
 12. Thecommunication network according to claim 10, wherein each of said nodesare linked by a link group consisting of a plurality of links, and whenthe first paths pairing up with said respective second path have anoverlapping risk sharing resource group, said plurality of second pathshaving the overlapping route share no link on said link group.
 13. Thecommunication network according to claim 8, wherein said first path is aprimary path, and said second path is an alternate path.
 14. Amanagement center connected to a plurality of nodes constituting anetwork, wherein said management center has information on a risksharing resource group, and when each of said nodes calculates a firstpath and a second path having different routes, the management centersends said information on the risk sharing resource group to said node.15. A management center connected to a plurality of nodes constituting anetwork, wherein said management center has currently set pathinformation, and when each of said nodes calculates a first path and asecond path having different routes, the management center refers tosaid path information to check whether an already set second pathoverlapping the second path calculated by said node exists.
 16. Themanagement center according to claim 14 connected to the plurality ofnodes constituting the network, wherein said management center has saidinformation on the risk sharing resource group and said pathinformation.
 17. A node constituting a network, wherein said node hastopology information of said network, and when calculating a first pathand a second path having different routes, said node obtains informationon a risk sharing resource group from a management center connected tosaid node.
 18. A node constituting a network, wherein said node hastopology information of said network, and when said node calculates afirst path and a second path having different routes, a managementcenter connected to said node refers to currently set path informationto check whether an already set second path overlapping the second pathcalculated by said node exists.
 19. The node according to claim 17constituting the network, wherein said node has the topology informationof said network, and the management center connected to said node hassaid information on the risk sharing resource group and said pathinformation.
 20. A node constituting a network, wherein said node hastopology information of said network and information on a risk sharingresource group, and when said node calculates a first path and a secondpath having different routes, a management center connected to said noderefers to currently set path information to check whether an already setsecond path overlapping the second path calculated by said node exists.21. A node constituting a network, wherein said node has topologyinformation of said network, information on a risk sharing resourcegroup, and information on a currently set path passing the node itself,and when calculating a first path and a second path having differentroutes, said node refers to these information.
 22. A node in acommunication network in which a communication network as set forth inclaim 19 consists of a plurality of subnetworks, wherein each of saidnodes has topology information of said network, and comprises anexternal routing table showing a boundary node that a route passes whena path to a destination node in another subnetwork is set.
 23. A node ina communication network in which a communication network as set forth inclaim 21 consists of a plurality of subnetworks, wherein each of saidnodes has topology information of said network, information on a risksharing resource group, and currently set path information, andcomprises an external routing table showing a boundary node that a routepasses when a path to a destination node in another subnetwork is set.24. A path setting method in a communication network comprising aplurality of nodes constituting a network and a management centerconnected to each of said nodes, wherein each of said nodes has topologyinformation of said network, and said management center has informationon a risk sharing resource group, and said method comprises: a firststep in which a source node refers to said topology information of thenetwork to calculate a route of a first path and send the route obtainedto said management center; a second step in which said management centerrefers to said information on the risk sharing resource group to returna list of a link group not belonging to said risk sharing resource groupthat the route sent from said source node passes to said source node;and a third step in which said source node refers to the list sent fromsaid management center to calculate a route of a second path.
 25. A pathsetting method in a communication network comprising a plurality ofnodes constituting a network and a management center connected to eachof said nodes, wherein each of said nodes has topology information ofsaid network, and said management center has path information, and saidmethod comprises: a first step in which a source node refers to saidtopology information of the network to calculate routes of a first pathand a second path and send the routes obtained to said managementcenter; and a second step in which said management center refers to saidpath information to search for an existing second path having a routeoverlapping the route of said second path.
 26. The path setting methodaccording to claim 25, further comprising a third step in which when itis determined in said second step that an already set second path havinga route overlapping the route of said second path exists, saidmanagement center sends a message that a link can be shared in anoverlapping link group to said source node when said second path is set.27. The path setting method according to claim 25, further comprising afourth step in which when it is determined in said second step that analready set second path having a route overlapping the route of saidsecond path does not exist, said management center sends a message thata link can not be shared in an overlapping link group to said sourcenode when said second path is set.
 28. A path setting method in acommunication network comprising a plurality of nodes constituting anetwork and a management center connected to each of said nodes, whereineach of said nodes has topology information of said network, and saidmanagement center has information on a risk sharing resource group andcurrently set path information, and said method comprises: a first stepin which a source node refers to said topology information of thenetwork to calculate routes of a first path and a second path and sendthe routes obtained to said management center; a second step in whichsaid management center refers to said information on the risk sharingresource group to check risk sharing resource groups that the routes ofsaid first path and said second path sent from said source node pass;and a third step in which when it is determined in said second step thatsaid risk sharing resource groups do not overlap, said management centerrefers to said path information to search for an already set second pathhaving a route overlapping the route of said second path.
 29. The pathsetting method according to claim 28, further comprising a fourth stepin which when an already set second path having a route overlapping theroute of said second path exists in said third step, said risk sharingresource groups that the route of the first path corresponding to saidsecond path and a route of a first path corresponding to said alreadyset second path pass are checked.
 30. The path setting method accordingto claim 29, further comprising a fifth step in which when it isdetermined in said fourth step that said risk sharing resource groups ofboth of the first paths do not overlap, said management center sends amessage that a link can be shared in an overlapping link group to saidsource node when said second path is set.
 31. The path setting methodaccording to claim 29, further comprising a sixth step in which when itis determined in said fourth step that said risk sharing resource groupsof both of the first paths overlap, said management center sends amessage that a link can not be shared in an overlapping link group tosaid source node when said second path is set.
 32. A path setting methodin a communication network comprising a plurality of nodes constitutinga network and a management center connected to each of said nodes,wherein each of said nodes has topology information of said network, andsaid management center has information on a risk sharing resource groupand currently set path information, and said method comprises: a firststep in which a source node refers to said topology information of thenetwork to calculate a route of a first path and send the route obtainedto said management center; a second step in which said management centerrefers to said information on the risk sharing resource group to returna list of a link group not belonging to said risk sharing resource groupthat the route sent from said source node passes to said source node; athird step in which said source node refers to the list sent from saidmanagement center to calculate a route of a second path and send theroute obtained to said management center; and a fourth step in whichsaid management center refers to said path information to search for analready set second path having a route overlapping the route of saidsecond path.
 33. The path setting method according to claim 32, furthercomprising a fifth step in which when an already set second path havinga route overlapping the route of said second path exists in said fourthstep, said risk sharing resource groups that the route of the first pathcorresponding to said second path and a route of a first pathcorresponding to said already set second path pass are checked.
 34. Thepath setting method according to claim 33, further comprising a sixthstep in which when it is determined in said fifth step that said risksharing resource groups of both of the first paths do not overlap, saidmanagement center sends a message that a link can be shared in anoverlapping link group to said source node when said second path is set.35. The path setting method according to claim 33, further comprising aseventh step in which when it is determined in said fifth step that saidrisk sharing resource groups of both of the first paths overlap, saidmanagement center sends a message that a link can not be shared in anoverlapping link group to said source node when said second path is set.36. A path setting method in a communication network comprising aplurality of nodes constituting a network and a management centerconnected to each of said nodes, wherein each of said nodes has topologyinformation of said network and information on a risk sharing resourcegroup, and said management center has currently set path information,and said method comprises: a first step in which a source node refers tosaid topology information of the network and said information on therisk sharing resource group to calculate routes of a first path and asecond path so as not to pass the same risk sharing resource group; asecond step in which said source node sends the routes calculated andobtained to said management center; and a third step in which saidmanagement center refers to said path information to search for analready set second path having a route overlapping the route of saidsecond path.
 37. The path setting method according to claim 36, furthercomprising a fourth step in which when an already set second path havinga route overlapping the route of said second path exists in said thirdstep, said risk sharing resource groups that the route of the first pathcorresponding to said second path and a route of a first pathcorresponding to said already set second path pass are checked.
 38. Thepath setting method according to claim 37, further comprising a fifthstep in which when it is determined in said fourth step that said risksharing resource groups of both of the first paths do not overlap, saidmanagement center sends a message that a link can be shared in anoverlapping link group to said source node when said second path is set.39. A path setting method in a communication network comprising aplurality of nodes constituting a network, wherein each of said nodeshas topology information of said network, information on a risk sharingresource group, and information on a currently set path passing the nodeitself, and said method comprises: a first step in which a source noderefers to said topology information of the network and said informationon the risk sharing resource group to calculate routes of a first pathand a second path so as not to pass the same risk sharing resourcegroup; and a second step in which each node on said routes receives asignal from an upstream node, refers to said information on the risksharing resource group and said information on the currently set pathpassing the node itself to detect a second path having an overlappingroute in a link group between the node itself and a downstream node andcompare said risk sharing resource groups of said first path.
 40. A pathsetting method in a communication network in which a communicationnetwork as set forth in claims 32 consists of a plurality ofsubnetworks, wherein each of said nodes comprises an external routingtable showing a boundary node that a route passes when a path to adestination node in another subnetwork is set, and said methodcomprises: a ninth step in which a first path and a second path from asource node to said boundary node are set; and a tenth step in which afirst path and a second path from said boundary node to a destinationnode in another subnetwork are set.
 41. A path setting method in acommunication network in which a communication network as set forth inclaim 39 consists of a plurality of subnetworks, wherein each of saidnode comprises an external routing table showing a boundary node that aroute passes when a path to a destination node in another subnetwork isset, and said method comprises: a third step in which a first path and asecond path from a source node to said boundary node are set; and afourth step in which a first path and a second path from said boundarynode to a destination node in another subnetwork are set.
 42. The pathsetting method according to claim 24, wherein said first path is aprimary path, and set second path is an alternate path.
 43. A recordingmedium storing a path setting program for controlling a node in acommunication network comprising a plurality of nodes constituting anetwork and a management center connected to each of the nodes, whereineach of the nodes has topology information of the network, and themanagement center has information on a risk sharing resource group, andsaid path setting program comprises: a first step in which a source noderefers to said topology information of the network to calculate a routeof a first path and send the route obtained to said management center,and a second step in which said management center refers to saidinformation on the risk sharing resource group to return a list of alink group not belonging to said risk sharing resource group that theroute sent from said source node passes to said source node, and thensaid source node refers to the list sent from said management center tocalculate a route of a second path.
 44. A recording medium storing apath setting program for controlling a node in a communication networkcomprising a plurality of nodes constituting a network and a managementcenter connected to each of said nodes, wherein each of said nodes hastopology information of said network, and said management center haspath information, and said path setting program comprises a first stepin which a source node refers to said topology information of thenetwork to calculate routes of a first path and a second path and sendthe routes obtained to said management center, and said managementcenter refers to said path information to search for an existing secondpath having a route overlapping the route of said second path.
 45. Therecording medium according to claim 44, storing said path settingprogram for controlling a node, wherein said path setting program isdesigned such that when it is determined that an already set second pathhaving a route overlapping the route of said second path exists, saidmanagement center sends a message that a link can be shared in anoverlapping link group to said source node when said second path is set,and then a link allocated to the already set second path having theroute overlapping the route of said second path is allocated in saidlink group.
 46. The recording medium according to claim 44, storing saidpath setting program for controlling a node, wherein said path settingprogram is designed such that when it is determined that an already setsecond path having a route overlapping the route of said second pathdoes not exist, said management center sends a message that a link cannot be shared in an overlapping link group to said source node when saidsecond path is set, and then a link other than a link allocated to thealready set second path having the route overlapping the route of saidsecond path is allocated in said link group.
 47. A recording mediumstoring a path setting program for controlling a node in a communicationnetwork comprising a plurality of nodes constituting a network and amanagement center connected to each of said nodes, wherein each of saidnodes has topology information of said network, and said managementcenter has information on a risk sharing resource group and currentlyset path information, said path setting program comprises a first stepin which a source node refers to said topology information of thenetwork to calculate routes of a first path and a second path and sendthe routes obtained to said management center, and said managementcenter refers to said information on the risk sharing resource group tocheck risk sharing resource groups that the routes of said first pathand said second path sent from said source node passes, and when it isdetermined that said risk sharing resource groups do not overlap, saidmanagement center refers to said path information to search for analready set second path having a route overlapping the route of saidsecond path.
 48. The recording medium according to claim 47, storingsaid path setting program for controlling a node, wherein said pathsetting program is designed such that when an already set second pathhaving a route overlapping the route of said second path exists, saidrisk sharing resource groups that the route of the first pathcorresponding to said second path and a route of a first pathcorresponding to said already set second path pass are checked.
 49. Therecording medium according to claim 48, storing said path settingprogram for controlling a node, wherein said path setting program isdesigned such that when it is determined that said risk sharing resourcegroups of both of said first paths do not overlap, said managementcenter sends a message that a link can be shared in an overlapping linkgroup to said source node when said second path is set, and then a linkallocated to an already set second path having a route overlapping theroute of said second path is allocated in said link group.
 50. Therecording medium according to claim 48, storing said path settingprogram for controlling a node, wherein said path setting program isdesigned such that when it is determined that said risk sharing resourcegroups of both of said first paths overlap, said management center sendsa message that a link can not be shared in an overlapping link group tosaid source node when said second path is set, and then a link otherthan a link allocated to an already set second path having a routeoverlapping the route of said second path is allocated in said linkgroup.
 51. A recording medium storing a path setting program forcontrolling a node in a communication network comprising a plurality ofnodes constituting a network and a management center connected to eachof the nodes, wherein each of said nodes has topology information ofsaid network, and said management center has information on a risksharing resource group and currently set path information, and said pathsetting program comprises: a first step in which a source node refers tosaid topology information of the network to calculate a route of a firstpath and send the route obtained to said management center; and a secondstep in which said management center refers to said information on therisk sharing resource group to return a list of a link group notbelonging to said risk sharing resource group that the route sent fromsaid source node passes to said source node, and then said source noderefers to the list sent from said management center to calculate a routeof a second path and send the route obtained to said management center,and said management center refers to said path information to search foran already set second path having a route overlapping the route of saidsecond path.
 52. The recording medium according to claim 51, storingsaid path setting program for controlling a node, wherein said pathsetting program is designed such that when an already set second pathhaving a route overlapping the route of said second path exists, saidrisk sharing resource groups that the route of the first pathcorresponding to said second path and a route of a first pathcorresponding to said already set second path pass are checked.
 53. Therecording medium according to claim 52, storing said path settingprogram for controlling a node, wherein said path setting program isdesigned such that when it is determined that said risk sharing resourcegroups of both of said first paths do not overlap, said managementcenter sends a message that a link is shared in an overlapping linkgroup to said source node when said second path is set, and then a linkallocated to an already set second path having a route overlapping theroute of said second path is allocated in said link group.
 54. Therecording medium according to claim 52, storing said path settingprogram for controlling a node, wherein said path setting program isdesigned such that when it is determined that said risk sharing resourcegroups of both of said first paths overlap, said management center sendsa message that a link can not be shared in an overlapping link group tosaid source node when said second path is set, and then a link otherthan a link allocated to an already set second path having a routeoverlapping the route of said second path is allocated in said linkgroup.
 55. A recording medium storing a path setting program forcontrolling a node in a communication network comprising a plurality ofnodes constituting a network and a management center connected to eachof said nodes, wherein each of said nodes has topology information ofsaid network and information on a risk sharing resource group, and saidmanagement center has currently set path information, and said pathsetting program comprises: a first step in which a source node refers tosaid topology information of the network and said information on therisk sharing resource group to calculate routes of a first path and asecond path so as not to pass the same risk sharing resource group; anda second step in which said source node sends the routes calculated andobtained to said management center, and said management center refers tosaid path information to search for an already set second path having aroute overlapping the route of said second path.
 56. The recordingmedium according to claim 55, storing said path setting program forcontrolling a node, wherein said path setting program is designed suchthat when an already set second path having a route overlapping theroute of said second path exists, said risk sharing resource group thatthe route of the first path corresponding to said second path and aroute of a first path corresponding to said already set second path passare checked.
 57. The recording medium according to claim 56, storingsaid path setting program for controlling a node, wherein said pathsetting program is designed such that when it is determined that saidrisk sharing resource groups do not overlap, said management centersends a message that a link can be shared in an overlapping link groupto said source node when said second path is set, and then a linkallocated to an already set second path having a route overlapping theroute of said second path is allocated in said link group.
 58. Arecording medium storing a path setting program for controlling a nodein a communication network comprising a plurality of nodes constitutinga network, wherein each of said nodes has topology information of saidnetwork, information on a risk sharing resource group, and informationon a currently set path passing the node itself, and said path settingprogram comprises: a first step in which a source node refers to saidtopology information of the network and said information on the risksharing resource group to calculate routes of a first path and a secondpath so as not to pass the same risk sharing resource group; and asecond step in which each node on said routes receives a signal from anupstream node, refers to said information on the risk sharing resourcegroup and said information on the currently set path passing the nodeitself to detect a second path having an overlapping route in a linkgroup between the node itself and a downstream node and compare saidrisk sharing resource groups of said first path.
 59. A recording mediumstoring a path setting program for controlling a node in a communicationnetwork consisting of a plurality of subnetworks, said subnetworks arenetworks defined in claim 51, wherein each of said nodes comprises anexternal routing table showing a boundary node that a route passes whena path to a destination node in another subnetwork is set, and said pathsetting program comprises: a third step in which a first path and asecond path from a source node to said boundary node are set; and afourth step in which a first path and a second path from said boundarynode to a destination node in another subnetwork are set.
 60. Arecording medium storing a path setting program for controlling a nodein a communication network consisting of a plurality of subnetworks,said subnetworks are networks defined in claim 58, wherein each of saidnode comprises an external routing table showing a boundary node that aroute passes when a path to a destination node in another subnetwork isset, and said path setting program comprises: a third step in which afirst path and a second path from a source node to said boundary nodeare set; and a fourth step in which a first path and a second path fromsaid boundary node to a destination node in another subnetwork are set.61. The recording medium according to claim 43, storing said pathsetting program for controlling a node, wherein said path settingprogram is designed such that said first path is a primary path, and setsecond path is an alternate path.
 62. A recording medium having a pathsetting program recorded thereon for a management center in acommunication network comprising a plurality of nodes constituting anetwork and a management center connected to each of the nodes, whereineach of said nodes has topology information of the network, and saidmanagement center has information on a risk sharing resource group, saidmedium comprises a first step in which a source node refers to saidtopology information of the network to calculate a route of a first pathand send the route obtained to said management center, and then saidmanagement center refers to said information on the risk sharingresource group to return a list of a link group not belonging to saidrisk sharing resource group that the route sent from said source nodepasses to said source node, and said source node refers to the list sentfrom said management center to calculate a route of a second path.
 63. Arecording medium having a path setting program recorded thereon for amanagement center in a communication network comprising a plurality ofnodes constituting a network and a management center connected to eachof said nodes, wherein each of said nodes has topology information ofsaid network, and said management center has path information, and saidpath setting program comprises a first step in which a source noderefers to said topology information of the network to calculate routesof a first path and a second path and send the routes obtained to saidmanagement center, and then said management center refers to said pathinformation to search for an existing second path having a routeoverlapping the route of said second path.
 64. The recording mediumaccording to claim 63, storing said path setting program for amanagement center, wherein said path setting program is designed furthercomprises a second step in which when it is determined in said firststep that an already set second path having a route overlapping theroute of said second path exists, said management center sends a messagethat a link can be shared in an overlapping link group to said sourcenode when said second path is set.
 65. The recording medium according toclaim 63, storing said a path setting program for a management center,wherein said path setting program comprises a third step in which whenit is determined in said first step that a second path having a routeoverlapping the route of said second path does not exist, saidmanagement center sends a message that a link can not be shared to saidsource node when said second path is set.
 66. A recording medium storinga path setting program for a management center in a communicationnetwork comprising a plurality of nodes constituting a network and amanagement center connected to each of said nodes, wherein each of saidnodes has topology information of said network, and said managementcenter has information on a risk sharing resource group and currentlyset path information, and said path setting program comprises: a firststep in which a source node refers to said topology information of thenetwork to calculate routes of a first path and a second path and sendthe routes obtained to said management center, and then said managementcenter refers to said information on the risk sharing resource group tocheck risk sharing resource groups that the routes of said first pathand said second path sent from said source node pass; and a second stepin which when it is determined in said first step that said risk sharingresource groups do not overlap, said management center refers to saidpath information to search for an already set second path having a routeoverlapping the route of said second path.
 67. The recording mediumaccording to claim 66, storing said path setting program for amanagement center, wherein said path setting program further comprises athird step in which when an already set second path having a routeoverlapping the route of said second path exists in said second step,said risk sharing resource groups that the route of the first pathcorresponding to said second path and a route of a first pathcorresponding to said already set second path pass are checked.
 68. Therecording medium according to claim 67, storing said path settingprogram for a management center, wherein said path setting programfurther comprises a fourth step in which when it is determined in saidthird step that said risk sharing resource groups of both of the firstpaths do not overlap, said management center sends a message that a linkcan be shared in an overlapping link group to said source node when saidsecond path is set.
 69. The recording medium according to claim 67,storing said path setting program for a management center, wherein saidpath setting program further comprises a fifth step in which when it isdetermined in said third step that said risk sharing resource groups ofboth of the first paths overlap, said management center sends a messagethat a link can not be shared in an overlapping link group to saidsource node when said second path is set.
 70. A recording medium storinga path setting program for a management center in a communicationnetwork comprising a plurality of nodes constituting a network and amanagement center connected to each of said nodes, wherein each of saidnodes has topology information of said network, and said managementcenter has information on a risk sharing resource group and currentlyset path information, and said path setting program comprises: a firststep in which a source node refers to said topology information of thenetwork to calculate a route of a first path and send the route obtainedto said management center, and then said management center refers tosaid information on the risk sharing resource group to return a list ofa link group not belonging to said risk sharing resource group that theroute sent from said source node passes to said source node; and asecond step in which said source node refers to the list sent from saidmanagement center to calculate a route of a second path and send theroute obtained to said management center, and then said managementcenter refers to said path information to search for an already setsecond path having a route overlapping the route of said second path.71. The recording medium according to claim 70, storing said pathsetting program for a management center, wherein said path settingprogram further comprises a third step in which when an already setsecond path having a route overlapping the route of said second pathexists in said second step, said risk sharing resource groups that theroute of the first path corresponding to said second path and a route ofa first path corresponding to said already set second path pass arechecked.
 72. The recording medium according to claim 71, storing saidpath setting program for a management center, wherein said path settingprogram further comprises a fourth step in which when it is determinedin said third step that said risk sharing resource groups of both of thefirst paths do not overlap, said management center sends a message thata link can be shared in an overlapping link group to said source nodewhen said second path is set.
 73. The recording medium according toclaim 71, storing said path setting program for a management center,wherein said path setting program further comprises a fifth step inwhich when it is determined in said third step that said risk sharingresource groups of both of the first paths overlap, said managementcenter sends a message that a link can not be shared in an overlappinglink group to said source node when said second path is set.
 74. Arecording medium storing a path setting program for a management centerin a communication network comprising a plurality of nodes constitutinga network and a management center connected to each of the nodes,wherein each of said nodes has topology information of said network andinformation on a risk sharing resource group, and said management centerhas currently set path information, and said path setting programcomprises a first step in which a source node refers to said topologyinformation of the network and said information on the risk sharingresource group to calculate routes of a first path and a second path soas not to pass the same risk sharing resource group and send the routesobtained to said management center, and then said management centerrefers to said path information to search for an already set second pathhaving a route overlapping the route of said second path.
 75. Therecording medium according to claim 74, storing said path settingprogram for a management center, wherein said path setting programfurther comprises a second step in which when an already set second pathhaving a route overlapping the route of said second path exists in saidfirst step, said risk sharing resource groups that the route of thefirst path corresponding to said second path and a route of a first pathcorresponding to said already set second path pass are checked.
 76. Therecording medium according to claim 75, storing said path settingprogram for a management center, wherein said path setting programfurther comprises a third step in which when it is determined in saidsecond step that said risk sharing resource groups of both of the firstpaths do not overlap, said management center sends a message that a linkcan be shared in an overlapping link group to said source node when saidsecond path is set.
 77. A recording medium storing a path settingprogram for a management center in a communication network consisting ofa plurality of subnetworks, said subnetworks are networks defined inclaim 70, wherein each of said nodes comprises an external routing tableshowing a boundary node that a route passes when a path to a destinationnode in another subnetwork is set, and said setting program comprises: asixth step in which a first path and a second path from a source node tosaid boundary node are set; and a seventh step in which a first path anda second path from said boundary node to a destination node in anothersubnetwork are set.
 78. The recording medium according to claim 62,storing said path setting program for a management center, wherein saidpath setting program is designed such that said first path is a primarypath, and set second path is an alternate path.
 79. A path settingmethod according to claim 39, wherein in said second step, signalingmessage to set said second path includes the identity information of allof the risk sharing resource groups through which said first pathpasses.
 80. A recording medium according to claim 58, wherein in saidsecond step, signaling message to set said second path includes theidentity information of all of the risk sharing resource groups throughwhich said first path passes.
 81. A nod e according to claim 21, whereinsaid node, next to said calculating a first path and a second path,receives signaring message from an upstream node including the identityinformtaion of all of the risk sharing resource groups through whichsaid first path passes to detect a second path having an overlappingroute in a link group betweeen the node itself and a downstream node andcompare said risk sharing resoure groups of said first path.